diff --git a/key_cacher.go b/key_cacher.go
index cc088f9..2a5e00e 100644
--- a/key_cacher.go
+++ b/key_cacher.go
@@ -120,6 +120,7 @@ type MemoryKeyCacher struct {
 	maxKeyAge    time.Duration
 	maxCacheSize int
 	keyIDGetter  KeyIDGetter
+	mu           *sync.RWMutex
 }
 
 type keyCacherEntry struct {
@@ -160,24 +161,20 @@ func (gkc *GMemoryKeyCacher) Get(keyID string) (*jose.JSONWebKey, error) {
 
 // NewMemoryKeyCacher creates a new Keycacher interface with option
 // to set max age of cached keys and max size of the cache.
-func NewMemoryKeyCacher(maxKeyAge time.Duration, maxCacheSize int, keyIdentifyStrategy string) KeyCacher {
+func NewMemoryKeyCacher(maxKeyAge time.Duration, maxCacheSize int, keyIdentifyStrategy string) *MemoryKeyCacher {
 	return &MemoryKeyCacher{
 		entries:      map[string]keyCacherEntry{},
 		maxKeyAge:    maxKeyAge,
 		maxCacheSize: maxCacheSize,
 		keyIDGetter:  KeyIDGetterFactory(keyIdentifyStrategy),
+		mu:           new(sync.RWMutex),
 	}
 }
 
-func NewGlobalMemoryKeyCacher(maxKeyAge time.Duration, maxCacheSize int, keyIdentifyStrategy string) KeyCacher {
+func NewGlobalMemoryKeyCacher(maxKeyAge time.Duration, maxCacheSize int, keyIdentifyStrategy string) *GMemoryKeyCacher {
 	kc := &GMemoryKeyCacher{
-		MemoryKeyCacher: &MemoryKeyCacher{
-			entries:      map[string]keyCacherEntry{},
-			maxKeyAge:    maxKeyAge,
-			maxCacheSize: maxCacheSize,
-			keyIDGetter:  KeyIDGetterFactory(keyIdentifyStrategy),
-		},
-		Global: GlobalCacher{},
+		MemoryKeyCacher: NewMemoryKeyCacher(maxKeyAge, maxCacheSize, keyIdentifyStrategy),
+		Global:          GlobalCacher{},
 	}
 	if keyIdentifyStrategy == "" {
 		keyIdentifyStrategy = defaultStrategy
@@ -191,7 +188,9 @@ func NewGlobalMemoryKeyCacher(maxKeyAge time.Duration, maxCacheSize int, keyIden
 
 // Get obtains a key from the cache, and checks if the key is expired
 func (mkc *MemoryKeyCacher) Get(keyID string) (*jose.JSONWebKey, error) {
+	mkc.mu.RLock()
 	searchKey, ok := mkc.entries[keyID]
+	mkc.mu.RUnlock()
 	if ok {
 		if mkc.maxKeyAge == MaxKeyAgeNoCheck || !mkc.keyIsExpired(keyID) {
 			return &searchKey.JSONWebKey, nil
@@ -212,18 +211,22 @@ func (mkc *MemoryKeyCacher) Add(keyID string, downloadedKeys []jose.JSONWebKey)
 			addingKeyID = cacheKey
 		}
 		if mkc.maxCacheSize == -1 {
+			mkc.mu.Lock()
 			mkc.entries[cacheKey] = keyCacherEntry{
 				addedAt:    time.Now(),
 				JSONWebKey: downloadedKeys[i],
 			}
+			mkc.mu.Unlock()
 		}
 	}
 	if addingKey.Key != nil {
 		if mkc.maxCacheSize != -1 {
+			mkc.mu.Lock()
 			mkc.entries[addingKeyID] = keyCacherEntry{
 				addedAt:    time.Now(),
 				JSONWebKey: addingKey,
 			}
+			mkc.mu.Unlock()
 			mkc.handleOverflow()
 		}
 		return &addingKey, nil
@@ -233,8 +236,14 @@ func (mkc *MemoryKeyCacher) Add(keyID string, downloadedKeys []jose.JSONWebKey)
 
 // keyIsExpired deletes the key from cache if it is expired
 func (mkc *MemoryKeyCacher) keyIsExpired(keyID string) bool {
-	if time.Now().After(mkc.entries[keyID].addedAt.Add(mkc.maxKeyAge)) {
+	mkc.mu.RLock()
+	entry := mkc.entries[keyID].addedAt.Add(mkc.maxKeyAge)
+	mkc.mu.RUnlock()
+
+	if time.Now().After(entry) {
+		mkc.mu.Lock()
 		delete(mkc.entries, keyID)
+		mkc.mu.Unlock()
 		return true
 	}
 	return false
@@ -243,6 +252,7 @@ func (mkc *MemoryKeyCacher) keyIsExpired(keyID string) bool {
 // handleOverflow deletes the oldest key from the cache if overflowed
 func (mkc *MemoryKeyCacher) handleOverflow() {
 	if mkc.maxCacheSize < len(mkc.entries) {
+		mkc.mu.RLock()
 		var oldestEntryKeyID string
 		latestAddedTime := time.Now()
 		for entryKeyID := range mkc.entries {
@@ -251,6 +261,10 @@ func (mkc *MemoryKeyCacher) handleOverflow() {
 				oldestEntryKeyID = entryKeyID
 			}
 		}
+		mkc.mu.RUnlock()
+
+		mkc.mu.Lock()
 		delete(mkc.entries, oldestEntryKeyID)
+		mkc.mu.Unlock()
 	}
 }