diff --git a/util.go b/util.go
index 52116ac..c6b3bd7 100644
--- a/util.go
+++ b/util.go
@@ -37,39 +37,42 @@ func (pe ConfigParseError) Unwrap() error {
 	return pe.err
 }
 
-// toCaseInsensitiveValue checks if the value is a  map;
-// if so, create a copy and lower-case the keys recursively.
-func toCaseInsensitiveValue(value any) any {
+// CopyMap returns a deep copy of a map[any]any or map[string]any.  If value is
+// not one of those map types, then it is returned as-is.  If preserveCase is
+// false, then all keys will be converted to lower-case in the copy that is
+// returned.
+func CopyMap(value any, preserveCase bool) any {
+	var copyMap func(map[string]any, bool) map[string]any
+	copyMap = func(m map[string]any, preserveCase bool) map[string]any {
+		nm := make(map[string]any)
+
+		for key, val := range m {
+			if !preserveCase {
+				key = strings.ToLower(key)
+			}
+			switch v := val.(type) {
+			case map[any]any:
+				nm[key] = copyMap(cast.ToStringMap(v), preserveCase)
+			case map[string]any:
+				nm[key] = copyMap(v, preserveCase)
+			default:
+				nm[key] = v
+			}
+		}
+
+		return nm
+	}
+
 	switch v := value.(type) {
 	case map[any]any:
-		value = copyAndInsensitiviseMap(cast.ToStringMap(v))
+		value = copyMap(cast.ToStringMap(v), preserveCase)
 	case map[string]any:
-		value = copyAndInsensitiviseMap(v)
+		value = copyMap(v, preserveCase)
 	}
 
 	return value
 }
 
-// copyAndInsensitiviseMap behaves like insensitiviseMap, but creates a copy of
-// any map it makes case insensitive.
-func copyAndInsensitiviseMap(m map[string]any) map[string]any {
-	nm := make(map[string]any)
-
-	for key, val := range m {
-		lkey := strings.ToLower(key)
-		switch v := val.(type) {
-		case map[any]any:
-			nm[lkey] = copyAndInsensitiviseMap(cast.ToStringMap(v))
-		case map[string]any:
-			nm[lkey] = copyAndInsensitiviseMap(v)
-		default:
-			nm[lkey] = v
-		}
-	}
-
-	return nm
-}
-
 func insensitiviseVal(val any) any {
 	switch v := val.(type) {
 	case map[any]any:
diff --git a/util_test.go b/util_test.go
index 8d0bda8..d922947 100644
--- a/util_test.go
+++ b/util_test.go
@@ -19,7 +19,7 @@ import (
 	"github.com/stretchr/testify/assert"
 )
 
-func TestCopyAndInsensitiviseMap(t *testing.T) {
+func TestCopyMap(t *testing.T) {
 	var (
 		given = map[string]any{
 			"Foo": 32,
@@ -35,19 +35,54 @@ func TestCopyAndInsensitiviseMap(t *testing.T) {
 				"cde": "B",
 			},
 		}
+		expectedPreserveCase = map[string]any{
+			"Foo": 32,
+			"Bar": map[string]any{
+				"ABc": "A",
+				"cDE": "B",
+			},
+		}
 	)
 
-	got := copyAndInsensitiviseMap(given)
+	t.Run("convert to lower-case", func(t *testing.T) {
+		got := CopyMap(given, false)
 
-	assert.Equal(t, expected, got)
-	_, ok := given["foo"]
-	assert.False(t, ok)
-	_, ok = given["bar"]
-	assert.False(t, ok)
+		assert.Equal(t, expected, got)
+		_, ok := given["foo"]
+		assert.False(t, ok)
+		_, ok = given["bar"]
+		assert.False(t, ok)
 
-	m := given["Bar"].(map[any]any)
-	_, ok = m["ABc"]
-	assert.True(t, ok)
+		m := given["Bar"].(map[any]any)
+		_, ok = m["ABc"]
+		assert.True(t, ok)
+	})
+
+	t.Run("preserve case", func(t *testing.T) {
+		got := CopyMap(given, true)
+
+		assert.Equal(t, expectedPreserveCase, got)
+		_, ok := given["foo"]
+		assert.False(t, ok)
+		_, ok = given["bar"]
+		assert.False(t, ok)
+
+		m := given["Bar"].(map[any]any)
+		_, ok = m["ABc"]
+		assert.True(t, ok)
+	})
+
+	t.Run("not a map", func(t *testing.T) {
+		var (
+			given    = []any{42, "xyz"}
+			expected = []any{42, "xyz"}
+		)
+		got := CopyMap(given, false)
+		assert.Equal(t, expected, got)
+
+		got = CopyMap(given, true)
+		assert.Equal(t, expected, got)
+	})
 }
 
 func TestAbsPathify(t *testing.T) {
diff --git a/viper.go b/viper.go
index c1eab71..72bac3e 100644
--- a/viper.go
+++ b/viper.go
@@ -206,6 +206,10 @@ type Viper struct {
 	envKeyReplacer      StringReplacer
 	allowEmptyEnv       bool
 
+	// When caseSensitiveKeys is true, keys are preserved in their original
+	// case (i.e., not modified to lower-case).
+	caseSensitiveKeys bool
+
 	parents        []string
 	config         map[string]any
 	override       map[string]any
@@ -283,6 +287,20 @@ func EnvKeyReplacer(r StringReplacer) Option {
 	})
 }
 
+// CaseSensitiveKeys sets Viper to use case-sensitive keys, which preserves the
+// case of keys.  By default, all keys are converted to lower-case.
+//
+// Lookup keys (i.e., Get()) will always match environment variables in a
+// case-insensitive manner because Viper always converts the lookup key to
+// upper-case when searching for an environment variable.  The case of the
+// actual keys will be preserved, however, as seen in the output of AllKeys(),
+// AllSettings(), etc.
+func CaseSensitiveKeys(enable bool) Option {
+	return optionFunc(func(v *Viper) {
+		v.caseSensitiveKeys = enable
+	})
+}
+
 // NewWithOptions creates a new Viper instance.
 func NewWithOptions(opts ...Option) *Viper {
 	v := New()
@@ -706,7 +724,7 @@ func (v *Viper) searchIndexableWithPathPrefixes(source any, path []string) any {
 
 	// search for path prefixes, starting from the longest one
 	for i := len(path); i > 0; i-- {
-		prefixKey := strings.ToLower(strings.Join(path[0:i], v.keyDelim))
+		prefixKey := v.toLower(strings.Join(path[0:i], v.keyDelim))
 
 		var val any
 		switch sourceIndexable := source.(type) {
@@ -897,8 +915,7 @@ func GetViper() *Viper {
 func Get(key string) any { return v.Get(key) }
 
 func (v *Viper) Get(key string) any {
-	lcaseKey := strings.ToLower(key)
-	val := v.find(lcaseKey, true)
+	val := v.find(v.toLower(key), true)
 	if val == nil {
 		return nil
 	}
@@ -906,7 +923,7 @@ func (v *Viper) Get(key string) any {
 	if v.typeByDefValue {
 		// TODO(bep) this branch isn't covered by a single test.
 		valType := val
-		path := strings.Split(lcaseKey, v.keyDelim)
+		path := strings.Split(key, v.keyDelim)
 		defVal := v.searchMap(v.defaults, path)
 		if defVal != nil {
 			valType = defVal
@@ -950,14 +967,14 @@ func (v *Viper) Get(key string) any {
 func Sub(key string) *Viper { return v.Sub(key) }
 
 func (v *Viper) Sub(key string) *Viper {
-	subv := New()
 	data := v.Get(key)
 	if data == nil {
 		return nil
 	}
 
 	if reflect.TypeOf(data).Kind() == reflect.Map {
-		subv.parents = append(v.parents, strings.ToLower(key))
+		subv := NewWithOptions(CaseSensitiveKeys(v.caseSensitiveKeys))
+		subv.parents = append(v.parents, v.toLower(key))
 		subv.automaticEnvApplied = v.automaticEnvApplied
 		subv.envPrefix = v.envPrefix
 		subv.envKeyReplacer = v.envKeyReplacer
@@ -1196,7 +1213,7 @@ func (v *Viper) BindFlagValue(key string, flag FlagValue) error {
 	if flag == nil {
 		return fmt.Errorf("flag for %q is nil", key)
 	}
-	v.pflags[strings.ToLower(key)] = flag
+	v.pflags[v.toLower(key)] = flag
 	return nil
 }
 
@@ -1213,7 +1230,7 @@ func (v *Viper) BindEnv(input ...string) error {
 		return fmt.Errorf("missing key to bind to")
 	}
 
-	key := strings.ToLower(input[0])
+	key := v.toLower(input[0])
 
 	if len(input) == 1 {
 		v.env[key] = append(v.env[key], v.mergeWithEnvPrefix(key))
@@ -1243,12 +1260,13 @@ func (v *Viper) MustBindEnv(input ...string) {
 // Lastly, if no value was found and flagDefault is true, and if the key
 // corresponds to a flag, the flag's default value is returned.
 //
-// Note: this assumes a lower-cased key given.
-func (v *Viper) find(lcaseKey string, flagDefault bool) any {
+// Note: By default, this assumes that a lowercase key is given.
+// This behavior can be modified with viper.SetPreserveCase().
+func (v *Viper) find(key string, flagDefault bool) any {
 	var (
 		val    any
 		exists bool
-		path   = strings.Split(lcaseKey, v.keyDelim)
+		path   = strings.Split(key, v.keyDelim)
 		nested = len(path) > 1
 	)
 
@@ -1258,8 +1276,8 @@ func (v *Viper) find(lcaseKey string, flagDefault bool) any {
 	}
 
 	// if the requested key is an alias, then return the proper key
-	lcaseKey = v.realKey(lcaseKey)
-	path = strings.Split(lcaseKey, v.keyDelim)
+	key = v.realKey(key)
+	path = strings.Split(key, v.keyDelim)
 	nested = len(path) > 1
 
 	// Set() override first
@@ -1272,7 +1290,7 @@ func (v *Viper) find(lcaseKey string, flagDefault bool) any {
 	}
 
 	// PFlag override next
-	flag, exists := v.pflags[lcaseKey]
+	flag, exists := v.pflags[key]
 	if exists && flag.HasChanged() {
 		switch flag.ValueType() {
 		case "int", "int8", "int16", "int32", "int64":
@@ -1308,7 +1326,7 @@ func (v *Viper) find(lcaseKey string, flagDefault bool) any {
 
 	// Env override next
 	if v.automaticEnvApplied {
-		envKey := strings.Join(append(v.parents, lcaseKey), ".")
+		envKey := strings.Join(append(v.parents, key), ".")
 		// even if it hasn't been registered, if automaticEnv is used,
 		// check any Get request
 		if val, ok := v.getEnv(v.mergeWithEnvPrefix(envKey)); ok {
@@ -1318,7 +1336,7 @@ func (v *Viper) find(lcaseKey string, flagDefault bool) any {
 			return nil
 		}
 	}
-	envkeys, exists := v.env[lcaseKey]
+	envkeys, exists := v.env[key]
 	if exists {
 		for _, envkey := range envkeys {
 			if val, ok := v.getEnv(envkey); ok {
@@ -1360,7 +1378,7 @@ func (v *Viper) find(lcaseKey string, flagDefault bool) any {
 	if flagDefault {
 		// last chance: if no value is found and a flag does exist for the key,
 		// get the flag's default value even if the flag's value has not been set.
-		if flag, exists := v.pflags[lcaseKey]; exists {
+		if flag, exists := v.pflags[key]; exists {
 			switch flag.ValueType() {
 			case "int", "int8", "int16", "int32", "int64":
 				return cast.ToInt(flag.ValueString())
@@ -1457,8 +1475,7 @@ func stringToIntConv(val string) any {
 func IsSet(key string) bool { return v.IsSet(key) }
 
 func (v *Viper) IsSet(key string) bool {
-	lcaseKey := strings.ToLower(key)
-	val := v.find(lcaseKey, false)
+	val := v.find(v.toLower(key), false)
 	return val != nil
 }
 
@@ -1484,11 +1501,11 @@ func (v *Viper) SetEnvKeyReplacer(r *strings.Replacer) {
 func RegisterAlias(alias string, key string) { v.RegisterAlias(alias, key) }
 
 func (v *Viper) RegisterAlias(alias string, key string) {
-	v.registerAlias(alias, strings.ToLower(key))
+	v.registerAlias(alias, v.toLower(key))
 }
 
 func (v *Viper) registerAlias(alias string, key string) {
-	alias = strings.ToLower(alias)
+	alias = v.toLower(alias)
 	if alias != key && alias != v.realKey(key) {
 		_, exists := v.aliases[alias]
 
@@ -1533,11 +1550,9 @@ func (v *Viper) realKey(key string) string {
 func InConfig(key string) bool { return v.InConfig(key) }
 
 func (v *Viper) InConfig(key string) bool {
-	lcaseKey := strings.ToLower(key)
-
 	// if the requested key is an alias, then return the proper key
-	lcaseKey = v.realKey(lcaseKey)
-	path := strings.Split(lcaseKey, v.keyDelim)
+	key = v.realKey(v.toLower(key))
+	path := strings.Split(key, v.keyDelim)
 
 	return v.searchIndexableWithPathPrefixes(v.config, path) != nil
 }
@@ -1549,11 +1564,11 @@ func SetDefault(key string, value any) { v.SetDefault(key, value) }
 
 func (v *Viper) SetDefault(key string, value any) {
 	// If alias passed in, then set the proper default
-	key = v.realKey(strings.ToLower(key))
-	value = toCaseInsensitiveValue(value)
+	key = v.realKey(v.toLower(key))
+	value = CopyMap(value, v.caseSensitiveKeys)
 
 	path := strings.Split(key, v.keyDelim)
-	lastKey := strings.ToLower(path[len(path)-1])
+	lastKey := v.toLower(path[len(path)-1])
 	deepestMap := deepSearch(v.defaults, path[0:len(path)-1])
 
 	// set innermost value
@@ -1567,12 +1582,12 @@ func (v *Viper) SetDefault(key string, value any) {
 func Set(key string, value any) { v.Set(key, value) }
 
 func (v *Viper) Set(key string, value any) {
-	// If alias passed in, then set the proper override
-	key = v.realKey(strings.ToLower(key))
-	value = toCaseInsensitiveValue(value)
+	// If alias passed in, then set the proper default
+	key = v.realKey(v.toLower(key))
+	value = CopyMap(value, v.caseSensitiveKeys)
 
 	path := strings.Split(key, v.keyDelim)
-	lastKey := strings.ToLower(path[len(path)-1])
+	lastKey := v.toLower(path[len(path)-1])
 	deepestMap := deepSearch(v.override, path[0:len(path)-1])
 
 	// set innermost value
@@ -1661,8 +1676,10 @@ func (v *Viper) MergeConfigMap(cfg map[string]any) error {
 	if v.config == nil {
 		v.config = make(map[string]any)
 	}
-	insensitiviseMap(cfg)
-	mergeMaps(cfg, v.config, nil)
+	if !v.caseSensitiveKeys {
+		insensitiviseMap(cfg)
+	}
+	v.mergeMaps(cfg, v.config, nil)
 	return nil
 }
 
@@ -1761,7 +1778,9 @@ func (v *Viper) unmarshalReader(in io.Reader, c map[string]any) error {
 		}
 	}
 
-	insensitiviseMap(c)
+	if !v.caseSensitiveKeys {
+		insensitiviseMap(c)
+	}
 	return nil
 }
 
@@ -1783,10 +1802,10 @@ func (v *Viper) marshalWriter(f afero.File, configType string) error {
 	return nil
 }
 
-func keyExists(k string, m map[string]any) string {
-	lk := strings.ToLower(k)
+func (v *Viper) keyExists(k string, m map[string]any) string {
+	lk := v.toLower(k)
 	for mk := range m {
-		lmk := strings.ToLower(mk)
+		lmk := v.toLower(mk)
 		if lmk == lk {
 			return mk
 		}
@@ -1833,9 +1852,9 @@ func castMapFlagToMapInterface(src map[string]FlagValue) map[string]any {
 // instead of using a `string` as the key for nest structures beyond one level
 // deep. Both map types are supported as there is a go-yaml fork that uses
 // `map[string]any` instead.
-func mergeMaps(src, tgt map[string]any, itgt map[any]any) {
+func (v *Viper) mergeMaps(src, tgt map[string]any, itgt map[any]any) {
 	for sk, sv := range src {
-		tk := keyExists(sk, tgt)
+		tk := v.keyExists(sk, tgt)
 		if tk == "" {
 			v.logger.Debug("", "tk", "\"\"", fmt.Sprintf("tgt[%s]", sk), sv)
 			tgt[sk] = sv
@@ -1885,7 +1904,7 @@ func mergeMaps(src, tgt map[string]any, itgt map[any]any) {
 
 			ssv := castToMapStringInterface(tsv)
 			stv := castToMapStringInterface(ttv)
-			mergeMaps(ssv, stv, ttv)
+			v.mergeMaps(ssv, stv, ttv)
 		case map[string]any:
 			v.logger.Debug("merging maps")
 			tsv, ok := sv.(map[string]any)
@@ -1900,7 +1919,7 @@ func mergeMaps(src, tgt map[string]any, itgt map[any]any) {
 				)
 				continue
 			}
-			mergeMaps(tsv, ttv, nil)
+			v.mergeMaps(tsv, ttv, nil)
 		default:
 			v.logger.Debug("setting value")
 			tgt[tk] = sv
@@ -2063,7 +2082,7 @@ func (v *Viper) flattenAndMergeMap(shadow map[string]bool, m map[string]any, pre
 			m2 = cast.ToStringMap(val)
 		default:
 			// immediate value
-			shadow[strings.ToLower(fullKey)] = true
+			shadow[v.toLower(fullKey)] = true
 			continue
 		}
 		// recursively merge to shadow map
@@ -2089,7 +2108,7 @@ outer:
 			}
 		}
 		// add key
-		shadow[strings.ToLower(k)] = true
+		shadow[v.toLower(k)] = true
 	}
 	return shadow
 }
@@ -2108,7 +2127,7 @@ func (v *Viper) AllSettings() map[string]any {
 			continue
 		}
 		path := strings.Split(k, v.keyDelim)
-		lastKey := strings.ToLower(path[len(path)-1])
+		lastKey := v.toLower(path[len(path)-1])
 		deepestMap := deepSearch(m, path[0:len(path)-1])
 		// set innermost value
 		deepestMap[lastKey] = value
@@ -2204,3 +2223,13 @@ func (v *Viper) DebugTo(w io.Writer) {
 	fmt.Fprintf(w, "Config:\n%#v\n", v.config)
 	fmt.Fprintf(w, "Defaults:\n%#v\n", v.defaults)
 }
+
+// toLower returns a properly cased key based on the CaseSensitiveKeys option.
+// If preserveCase is true, then the unmodifed key is returned.  Otherwise, the
+// lower-cased key is returned.
+func (v *Viper) toLower(k string) string {
+	if v.caseSensitiveKeys {
+		return k
+	}
+	return strings.ToLower(k)
+}
diff --git a/viper_test.go b/viper_test.go
index 6b1b316..fdc6703 100644
--- a/viper_test.go
+++ b/viper_test.go
@@ -171,6 +171,41 @@ func initConfigs() {
 	unmarshalReader(r, v.config)
 }
 
+func initAllConfigs(v *Viper) {
+	var r io.Reader
+	v.SetConfigType("yaml")
+	r = bytes.NewReader(yamlExample)
+	v.unmarshalReader(r, v.config)
+
+	v.SetConfigType("json")
+	r = bytes.NewReader(jsonExample)
+	v.unmarshalReader(r, v.config)
+
+	v.SetConfigType("hcl")
+	r = bytes.NewReader(hclExample)
+	v.unmarshalReader(r, v.config)
+
+	v.SetConfigType("properties")
+	r = bytes.NewReader(propertiesExample)
+	v.unmarshalReader(r, v.config)
+
+	v.SetConfigType("toml")
+	r = bytes.NewReader(tomlExample)
+	v.unmarshalReader(r, v.config)
+
+	v.SetConfigType("env")
+	r = bytes.NewReader(dotenvExample)
+	v.unmarshalReader(r, v.config)
+
+	v.SetConfigType("json")
+	remote := bytes.NewReader(remoteExample)
+	v.unmarshalReader(remote, v.kvstore)
+
+	v.SetConfigType("ini")
+	r = bytes.NewReader(iniExample)
+	v.unmarshalReader(r, v.config)
+}
+
 func initConfig(typ, config string) {
 	Reset()
 	SetConfigType(typ)
@@ -482,6 +517,23 @@ func TestDefault(t *testing.T) {
 
 	assert.NoError(t, err)
 	assert.Equal(t, "leather", Get("clothing.jacket"))
+	assert.Equal(t, "leather", Get("clothing.Jacket"))
+}
+
+func TestDefault_CaseSensitive(t *testing.T) {
+	v := NewWithOptions(CaseSensitiveKeys(true))
+	v.SetDefault("age", 45)
+	assert.Equal(t, 45, v.Get("age"))
+
+	v.SetDefault("clothing.jacket", "slacks")
+	assert.Equal(t, "slacks", v.Get("clothing.jacket"))
+
+	v.SetConfigType("yaml")
+	err := v.ReadConfig(bytes.NewBuffer(yamlExample))
+
+	assert.NoError(t, err)
+	assert.Equal(t, "slacks", v.Get("clothing.jacket"))
+	assert.Equal(t, "leather", v.Get("clothing.Jacket"))
 }
 
 func TestUnmarshaling(t *testing.T) {
@@ -497,6 +549,29 @@ func TestUnmarshaling(t *testing.T) {
 	assert.Equal(t, []any{"skateboarding", "snowboarding", "go"}, Get("hobbies"))
 	assert.Equal(t, map[string]any{"jacket": "leather", "trousers": "denim", "pants": map[string]any{"size": "large"}}, Get("clothing"))
 	assert.Equal(t, 35, Get("age"))
+	// Lower-case key is found.
+	assert.Equal(t, true, Get("hacker"))
+	// Upper-case key is found.
+	assert.Equal(t, true, v.Get("Hacker"))
+}
+
+func TestUnmarshaling_CaseSensitive(t *testing.T) {
+	// Test preserving the case of keys read from a configuration
+	v := NewWithOptions(CaseSensitiveKeys(true))
+	v.SetConfigType("yaml")
+	v.ReadConfig(bytes.NewBuffer(yamlExample))
+
+	assert.True(t, v.InConfig("name"))
+	assert.True(t, v.InConfig("clothing.Jacket"))
+	assert.False(t, v.InConfig("state"))
+	assert.False(t, v.InConfig("clothing.hat"))
+	assert.Equal(t, "steve", v.Get("name"))
+	assert.Equal(t, []any{"skateboarding", "snowboarding", "go"}, v.Get("hobbies"))
+	assert.Equal(t, map[string]any{"Jacket": "leather", "trousers": "denim", "Pants": map[string]any{"size": "large"}}, v.Get("clothing"))
+	assert.Equal(t, 35, v.Get("age"))
+	assert.Equal(t, true, v.Get("Hacker"))
+	// Lower-case key is not found.
+	assert.Equal(t, nil, v.Get("hacker"))
 }
 
 func TestUnmarshalExact(t *testing.T) {
@@ -730,6 +805,36 @@ func TestEnvSubConfig(t *testing.T) {
 	assert.Equal(t, "large", subWithPrefix.Get("size"))
 }
 
+// This is an interesting case because the key passed to Viper.Get() is
+// converted to upper-case to perform a lookup in the environment variables.
+// So, this is a case for lookup where case-sensitivity is not strict.
+func TestEnvSubConfig_CaseSensitive(t *testing.T) {
+	v := NewWithOptions(CaseSensitiveKeys(true))
+	v.SetConfigType("yaml")
+	r := strings.NewReader(string(yamlExample))
+	if err := v.unmarshalReader(r, v.config); err != nil {
+		panic(err)
+	}
+
+	v.AutomaticEnv()
+
+	v.SetEnvKeyReplacer(strings.NewReplacer(".", "_"))
+
+	t.Setenv("CLOTHING_PANTS_SIZE", "small")
+	subv := v.Sub("clothing").Sub("Pants")
+	assert.Equal(t, "small", subv.Get("size"))
+	assert.Equal(t, "small", v.Get("clothing.Pants.size"))
+	assert.Equal(t, "small", v.Get("clothing.pants.size"))
+
+	// again with EnvPrefix
+	v.SetEnvPrefix("foo") // will be uppercased automatically
+	subWithPrefix := v.Sub("clothing").Sub("Pants")
+	t.Setenv("FOO_CLOTHING_PANTS_SIZE", "large")
+	assert.Equal(t, "large", subWithPrefix.Get("size"))
+	assert.Equal(t, "large", v.Get("clothing.Pants.size"))
+	assert.Equal(t, "large", v.Get("clothing.pants.size"))
+}
+
 func TestAllKeys(t *testing.T) {
 	initConfigs()
 
@@ -841,6 +946,118 @@ func TestAllKeys(t *testing.T) {
 	assert.Equal(t, all, AllSettings())
 }
 
+func TestAllKeys_CaseSensitive(t *testing.T) {
+	v := NewWithOptions(CaseSensitiveKeys(true))
+	initAllConfigs(v)
+
+	ks := []string{
+		"title",
+		"author.BIO",
+		"author.E-MAIL",
+		"author.GITHUB",
+		"author.NAME",
+		"newkey",
+		"owner.organization",
+		"owner.dob",
+		"owner.Bio",
+		"name",
+		"beard",
+		"ppu",
+		"batters.batter",
+		"hobbies",
+		"clothing.Jacket",
+		"clothing.trousers",
+		"DEFAULT.IMPORT_PATH",
+		"DEFAULT.NAME",
+		"DEFAULT.VERSION",
+		"clothing.Pants.size",
+		"age",
+		"Hacker",
+		"id",
+		"type",
+		"eyes",
+		"p_id",
+		"p_ppu",
+		"p_batters.batter.type",
+		"p_type",
+		"p_name",
+		"foos",
+		"TITLE_DOTENV",
+		"TYPE_DOTENV",
+		"NAME_DOTENV",
+	}
+	dob, _ := time.Parse(time.RFC3339, "1979-05-27T07:32:00Z")
+	all := map[string]any{
+		"owner": map[string]any{
+			"organization": "MongoDB",
+			"Bio":          "MongoDB Chief Developer Advocate & Hacker at Large",
+			"dob":          dob,
+		},
+		"title": "TOML Example",
+		"author": map[string]any{
+			"E-MAIL": "fake@localhost",
+			"GITHUB": "https://github.com/Unknown",
+			"NAME":   "Unknown",
+			"BIO":    "Gopher.\nCoding addict.\nGood man.\n",
+		},
+		"ppu":  0.55,
+		"eyes": "brown",
+		"clothing": map[string]any{
+			"trousers": "denim",
+			"Jacket":   "leather",
+			"Pants":    map[string]any{"size": "large"},
+		},
+		"DEFAULT": map[string]any{
+			"IMPORT_PATH": "gopkg.in/ini.v1",
+			"NAME":        "ini",
+			"VERSION":     "v1",
+		},
+		"id": "0001",
+		"batters": map[string]any{
+			"batter": []any{
+				map[string]any{"type": "Regular"},
+				map[string]any{"type": "Chocolate"},
+				map[string]any{"type": "Blueberry"},
+				map[string]any{"type": "Devil's Food"},
+			},
+		},
+		"Hacker": true,
+		"beard":  true,
+		"hobbies": []any{
+			"skateboarding",
+			"snowboarding",
+			"go",
+		},
+		"age":    35,
+		"type":   "donut",
+		"newkey": "remote",
+		"name":   "Cake",
+		"p_id":   "0001",
+		"p_ppu":  "0.55",
+		"p_name": "Cake",
+		"p_batters": map[string]any{
+			"batter": map[string]any{"type": "Regular"},
+		},
+		"p_type": "donut",
+		"foos": []map[string]any{
+			{
+				"foo": []map[string]any{
+					{"key": 1},
+					{"key": 2},
+					{"key": 3},
+					{"key": 4},
+				},
+			},
+		},
+		"TITLE_DOTENV": "DotEnv Example",
+		"TYPE_DOTENV":  "donut",
+		"NAME_DOTENV":  "Cake",
+	}
+
+	assert.ElementsMatch(t, ks, v.AllKeys())
+	assert.Equal(t, all, v.AllSettings())
+}
+
 func TestAllKeysWithEnv(t *testing.T) {
 	v := New()
 
@@ -1526,6 +1743,8 @@ func TestSub(t *testing.T) {
 
 	subv := v.Sub("clothing")
 	assert.Equal(t, v.Get("clothing.pants.size"), subv.Get("pants.size"))
+	assert.Equal(t, v.Get("clothing.jacket"), subv.Get("jacket"))
+	assert.Equal(t, v.Get("clothing.jacket"), subv.Get("Jacket"))
 
 	subv = v.Sub("clothing.pants")
 	assert.Equal(t, v.Get("clothing.pants.size"), subv.Get("size"))
@@ -1543,6 +1762,35 @@ func TestSub(t *testing.T) {
 	assert.Equal(t, []string{"clothing", "pants"}, subv.parents)
 }
 
+func TestSub_CaseSensitive(t *testing.T) {
+	v := NewWithOptions(CaseSensitiveKeys(true))
+	v.SetConfigType("yaml")
+	v.ReadConfig(bytes.NewBuffer(yamlExample))
+
+	subv := v.Sub("clothing")
+	assert.Equal(t, v.Get("clothing.pants.size"), subv.Get("pants.size"))
+	assert.Equal(t, v.Get("clothing.Jacket"), subv.Get("Jacket"))
+	assert.Equal(t, nil, subv.Get("jacket"))
+
+	subv = v.Sub("clothing.pants")
+	assert.Nil(t, subv)
+	subv = v.Sub("clothing.Pants")
+	//TODO: test Get with case-sensitive keys that are not found: "clothing.pants.size"
+	assert.Equal(t, v.Get("clothing.Pants.size"), subv.Get("size"))
+
+	subv = v.Sub("clothing.pants.size")
+	assert.Equal(t, (*Viper)(nil), subv)
+
+	subv = v.Sub("missing.key")
+	assert.Equal(t, (*Viper)(nil), subv)
+
+	subv = v.Sub("clothing")
+	assert.Equal(t, []string{"clothing"}, subv.parents)
+
+	subv = v.Sub("clothing").Sub("Pants")
+	assert.Equal(t, []string{"clothing", "Pants"}, subv.parents)
+}
+
 var hclWriteExpected = []byte(`"foos" = {
   "foo" = {
     "key" = 1
@@ -2046,6 +2294,44 @@ func TestMergeConfigMap(t *testing.T) {
 	assertFn(1234)
 }
 
+func TestMergeConfigMap_CaseSensitive(t *testing.T) {
+	v := NewWithOptions(CaseSensitiveKeys(true))
+	v.SetConfigType("yml")
+	err := v.ReadConfig(bytes.NewBuffer(yamlMergeExampleTgt))
+	require.NoError(t, err)
+
+	assertFn := func(i int) {
+		large := v.GetInt64("hello.largenum")
+		pop := v.GetInt("hello.pop")
+		assert.Equal(t, int64(765432101234567), large)
+		assert.Equal(t, i, pop)
+	}
+
+	assertFn(37890)
+
+	update := map[string]any{
+		"hello": map[string]any{
+			"Pop": 98,
+			"pop": 76,
+		},
+		"Hello": map[string]any{
+			"Pop": 1234,
+		},
+		"World": map[any]any{
+			"Rock": 345,
+		},
+	}
+
+	err = v.MergeConfigMap(update)
+	require.NoError(t, err)
+
+	assert.Equal(t, 345, v.GetInt("World.Rock"))
+	assert.Equal(t, 1234, v.GetInt("Hello.Pop"))
+
+	assert.Equal(t, 98, v.GetInt("hello.Pop"))
+	assertFn(76)
+}
+
 func TestUnmarshalingWithAliases(t *testing.T) {
 	v := New()
 	v.SetDefault("ID", 1)
@@ -2158,6 +2444,51 @@ R = 6
 	}
 }
 
+func TestCaseSensitive(t *testing.T) {
+	for _, config := range []struct {
+		typ     string
+		content string
+	}{
+		{"yaml", `
+aBcD: 1
+eF:
+  gH: 2
+  iJk: 3
+  Lm:
+    nO: 4
+    P:
+      Q: 5
+      R: 6
+`},
+		{"json", `{
+  "aBcD": 1,
+  "eF": {
+    "iJk": 3,
+    "Lm": {
+      "P": {
+        "Q": 5,
+        "R": 6
+      },
+      "nO": 4
+    },
+    "gH": 2
+  }
+}`},
+		{"toml", `aBcD = 1
+[eF]
+gH = 2
+iJk = 3
+[eF.Lm]
+nO = 4
+[eF.Lm.P]
+Q = 5
+R = 6
+`},
+	} {
+		doTestCaseSensitive(t, config.typ, config.content)
+	}
+}
+
 func TestCaseInsensitiveSet(t *testing.T) {
 	Reset()
 	m1 := map[string]any{
@@ -2197,6 +2528,58 @@ func TestCaseInsensitiveSet(t *testing.T) {
 	assert.True(t, ok)
 }
 
+func TestCaseSensitiveSet(t *testing.T) {
+	v := NewWithOptions(CaseSensitiveKeys(true))
+
+	m1 := map[string]any{
+		"Foo": 32,
+		"Bar": map[any]any{
+			"ABc": "A",
+			"cDE": "B",
+		},
+	}
+
+	m2 := map[string]any{
+		"Foo": 52,
+		"Bar": map[any]any{
+			"bCd": "A",
+			"eFG": "B",
+		},
+	}
+
+	v.Set("Given1", m1)
+	v.Set("Number1", 42)
+
+	v.SetDefault("Given2", m2)
+	v.SetDefault("Number2", 62)
+
+	// Verify SetDefault
+	assert.Equal(t, 62, v.Get("Number2"))
+	assert.Equal(t, nil, v.Get("number2"))
+	assert.Equal(t, 52, v.Get("Given2.Foo"))
+	assert.Equal(t, nil, v.Get("Given2.foo"))
+	assert.Equal(t, nil, v.Get("given2.Foo"))
+	assert.Equal(t, "A", v.Get("Given2.Bar.bCd"))
+	assert.Equal(t, nil, v.Get("Given2.Bar.bcd"))
+	assert.Equal(t, nil, v.Get("Given2.bar.bCd"))
+	assert.Equal(t, nil, v.Get("given2.Bar.bCd"))
+	_, ok := m2["Foo"]
+	assert.True(t, ok)
+
+	// Verify Set
+	assert.Equal(t, 42, v.Get("Number1"))
+	assert.Equal(t, nil, v.Get("number1"))
+	assert.Equal(t, 32, v.Get("Given1.Foo"))
+	assert.Equal(t, nil, v.Get("Given1.foo"))
+	assert.Equal(t, nil, v.Get("given1.Foo"))
+	assert.Equal(t, "A", v.Get("Given1.Bar.ABc"))
+	assert.Equal(t, nil, v.Get("Given1.Bar.abc"))
+	assert.Equal(t, nil, v.Get("Given1.bar.ABc"))
+	assert.Equal(t, nil, v.Get("given1.Bar.ABc"))
+	_, ok = m1["Foo"]
+	assert.True(t, ok)
+}
+
 func TestParseNested(t *testing.T) {
 	type duration struct {
 		Delay time.Duration
@@ -2237,6 +2620,30 @@ func doTestCaseInsensitive(t *testing.T, typ, config string) {
 	assert.Equal(t, 5, cast.ToInt(Get("ef.lm.p.q")))
 }
 
+func doTestCaseSensitive(t *testing.T, typ, config string) {
+	v := NewWithOptions(CaseSensitiveKeys(true))
+	v.SetConfigType(typ)
+
+	r := strings.NewReader(config)
+	if err := v.unmarshalReader(r, v.config); err != nil {
+		panic(err)
+	}
+
+	v.Set("RfD", true)
+	assert.Equal(t, nil, v.Get("rfd"))
+	assert.Equal(t, true, v.Get("RfD"))
+	assert.Equal(t, 0, cast.ToInt(v.Get("abcd")))
+	assert.Equal(t, 1, cast.ToInt(v.Get("aBcD")))
+	assert.Equal(t, 0, cast.ToInt(v.Get("ef.gh")))
+	assert.Equal(t, 2, cast.ToInt(v.Get("eF.gH")))
+	assert.Equal(t, 0, cast.ToInt(v.Get("ef.ijk")))
+	assert.Equal(t, 3, cast.ToInt(v.Get("eF.iJk")))
+	assert.Equal(t, 0, cast.ToInt(v.Get("ef.lm.no")))
+	assert.Equal(t, 4, cast.ToInt(v.Get("eF.Lm.nO")))
+	assert.Equal(t, 0, cast.ToInt(v.Get("ef.lm.p.q")))
+	assert.Equal(t, 5, cast.ToInt(v.Get("eF.Lm.P.Q")))
+}
+
 func newViperWithConfigFile(t *testing.T) (*Viper, string) {
 	watchDir := t.TempDir()
 	configFile := path.Join(watchDir, "config.yaml")
diff --git a/viper_yaml_test.go b/viper_yaml_test.go
index 264446b..1405ce8 100644
--- a/viper_yaml_test.go
+++ b/viper_yaml_test.go
@@ -7,9 +7,9 @@ hobbies:
     - snowboarding
     - go
 clothing:
-    jacket: leather
+    Jacket: leather
     trousers: denim
-    pants:
+    Pants:
         size: large
 age: 35
 eyes : brown