diff --git a/README.md b/README.md
index 7824074..08fc1a9 100644
--- a/README.md
+++ b/README.md
@@ -6,6 +6,6 @@ It is not production-ready. Use at your own risk.
 ## Features
 
 - Hashmaps - Thread-safe (MT Safe)
-- Dynamic strings - Not thread-safe (MT Unsafe)
+- Dynamic strings - Thread-safe (MT Safe)
   > Requires linking with the C math library (`-lm`)
 - Circular Queue - Not thread-safe (MT Unsafe)
diff --git a/string/mystring.c b/string/mystring.c
index 1d95df7..fbc3400 100644
--- a/string/mystring.c
+++ b/string/mystring.c
@@ -1,6 +1,7 @@
 #include "mystring.h"
 
 #include <math.h>
+#include <pthread.h>
 #include <stdlib.h>
 #include <string.h>
 
@@ -15,11 +16,21 @@ mcl_string *mcl_string_new(const char *text, long initial_capacity) {
 		return NULL;
 	}
 
+	/* Init pthread mutex */
+	int ret = pthread_mutex_init(&str->lock, NULL);
+	if (ret != 0) {
+		free(str);
+
+		return NULL;
+	}
+
 	/* Calculate size and capacity */
 	str->size = strlen(text);
 	size_t capacity = initial_capacity;
 
 	if (capacity != -1 && capacity - 1 < str->size) {
+		free(str);
+
 		return NULL;
 	}
 
@@ -50,9 +61,17 @@ int mcl_string_append(mcl_string *string, const char *text) {
 		return -1;
 	}
 
+	/* Lock resource */
+	int ret = pthread_mutex_lock(&string->lock);
+	if (ret != 0) {
+		return -1;
+	}
+
 	/* Handle empty case */
 	size_t text_len = strlen(text);
 	if (text_len == 0) {
+		pthread_mutex_unlock(&string->lock);
+
 		return 0;
 	}
 
@@ -64,6 +83,8 @@ int mcl_string_append(mcl_string *string, const char *text) {
 		/* Reallocate the buffer */
 		void *new_data = realloc(string->data, sizeof(char) * new_capacity);
 		if (!new_data) {
+			pthread_mutex_unlock(&string->lock);
+
 			return -1;
 		}
 
@@ -79,6 +100,12 @@ int mcl_string_append(mcl_string *string, const char *text) {
 	string->size = new_size;
 	string->data[string->size] = '\0';
 
+	/* Unlock resource */
+	ret = pthread_mutex_unlock(&string->lock);
+	if (ret != 0) {
+		return -1;
+	}
+
 	return 0;
 }
 
@@ -87,33 +114,68 @@ void mcl_string_free(mcl_string *string) {
 		return;
 	}
 
+	int ret = pthread_mutex_lock(&string->lock);
+	if (ret != 0) {
+		return;
+	}
+
 	if (string->data) {
 		free(string->data);
 	}
 
+	pthread_mutex_unlock(&string->lock);
+	pthread_mutex_destroy(&string->lock);
+
 	free(string);
 }
 
-size_t mcl_string_length(const mcl_string *string) {
+size_t mcl_string_length(mcl_string *string) {
 	if (string == NULL) {
 		return 0;
 	}
 
-	return string->size;
+	int ret = pthread_mutex_lock(&string->lock);
+	if (ret != 0) {
+		return 0;
+	}
+
+	size_t len = string->size;
+
+	pthread_mutex_unlock(&string->lock);
+
+	return len;
 }
 
-size_t mcl_string_capacity(const mcl_string *string) {
+size_t mcl_string_capacity(mcl_string *string) {
 	if (string == NULL) {
 		return 0;
 	}
 
-	return string->capacity;
+	int ret = pthread_mutex_lock(&string->lock);
+	if (ret != 0) {
+		return 0;
+	}
+
+	size_t cap = string->capacity;
+
+	pthread_mutex_unlock(&string->lock);
+
+	return cap;
 }
 
-const char *mcl_string_cstr(const mcl_string *string) {
+const char *mcl_string_cstr(mcl_string *string) {
 	if (string == NULL || string->data == NULL) {
 		return "";
 	}
 
-	return string->data;
+	int ret = pthread_mutex_lock(&string->lock);
+	if (ret != 0) {
+		return NULL;
+	}
+
+	char *data = string->data;
+
+	pthread_mutex_unlock(&string->lock);
+
+	return data;
 }
diff --git a/string/mystring.h b/string/mystring.h
index f6daa54..b13d40b 100644
--- a/string/mystring.h
+++ b/string/mystring.h
@@ -1,25 +1,27 @@
 #ifndef MYCLIB_STRING_H
 #define MYCLIB_STRING_H
 
+#include <pthread.h>
 #include <stddef.h>
 
 /**
- * @brief String structure
+ * @brief Thread-safe dynamic string structure
  */
 typedef struct mcl_string_t {
-	size_t size;	 /**< Length of the string (excluding null terminator) */
-	size_t capacity; /**< Total allocated capacity */
-	char *data;		 /**< Pointer to the string data */
+	size_t size;		  /**< Current length of the string (excluding null terminator) */
+	size_t capacity;	  /**< Allocated capacity */
+	char *data;			  /**< Pointer to string data */
+	pthread_mutex_t lock; /**< Mutex for thread safety */
 } mcl_string;
 
 /**
- * @brief Create a new string
+ * @brief Create a new string initialized with given text
  *
- * @param text The text to initialize from
- * @param initial_capacity The initial capacity, pass -1 to retrieve it from the text
- * @return Pointer to the new string, or NULL on failure
+ * @param text Initial text (cannot be NULL)
+ * @param initial_capacity Initial buffer capacity; pass -1 to auto-calculate
+ * @return Pointer to new string, or NULL on failure
  *
- * @note The caller is responsible for freeing the returned string with mcl_string_free() and to pass the right inital_capacity
+ * @note Caller must free the string with mcl_string_free()
  */
 mcl_string *mcl_string_new(const char *text, long initial_capacity);
 
@@ -27,47 +29,45 @@ mcl_string *mcl_string_new(const char *text, long initial_capacity);
  * @brief Append text to an existing string
  *
  * @param string The string to append to
- * @param text The string to append (can be empty)
+ * @param text Text to append (can be empty)
  * @return 0 on success, -1 on failure
  *
- * @note If it fails, the original string remains unchanged
+ * @note Original string remains unchanged if append fails
  */
 int mcl_string_append(mcl_string *string, const char *text);
 
 /**
- * @brief Free a string
+ * @brief Free a string and its resources
  *
- * @param string The string to free
- *
- * @note This function is safe to call with NULL pointers
+ * @param string String to free (NULL is safe)
  */
 void mcl_string_free(mcl_string *string);
 
 /**
  * @brief Get the current length of the string
  *
- * @param string The string to query
- * @return The length of the string, or 0 if string is NULL
+ * @param string String to query
+ * @return Length of string, or 0 if NULL
  */
-size_t mcl_string_length(const mcl_string *string);
+size_t mcl_string_length(mcl_string *string);
 
 /**
- * @brief Get the current capacity of the string
+ * @brief Get the current capacity of the string buffer
  *
- * @param string The string to query
- * @return The capacity of the string buffer, or 0 if string is NULL
+ * @param string String to query
+ * @return Capacity, or 0 if NULL
  */
-size_t mcl_string_capacity(const mcl_string *string);
+size_t mcl_string_capacity(mcl_string *string);
 
 /**
- * @brief Get a read-only string representation
+ * @brief Get a read-only C-string pointer
  *
- * @param string The string to access
- * @return Pointer to null-terminated string data, or empty string "" if string is NULL
+ * @param string String to access
+ * @return Null-terminated string pointer, or "" if NULL
  *
- * @warning The returned pointer should not be modified directly and may become
- *          invalid after any modification operation on the string
+ * @warning Do not modify returned pointer.
+ *          Pointer may become invalid after string modifications.
  */
-const char *mcl_string_cstr(const mcl_string *string);
+const char *mcl_string_cstr(mcl_string *string);
 
 #endif /* MYCLIB_STRING_H */