diff --git a/README.txt b/README.txt
index 9c4394d..6b19e41 100644
--- a/README.txt
+++ b/README.txt
@@ -22,3 +22,26 @@ int main() {
 
 Define DEBUG to initalize memory with OXFE, to detect uninitialized memory.
 Define DEBUG AND TK_MEMORY_TRACK to catch memory leaks when calling tk_check_memory_leaks().
+
+
+*** tk_strings.h ***
+```
+#define TK_STRINGS_IMPLEMENTATION
+#include "tk_strings.h"
+
+#include <stdlib.h>
+
+int main() {
+    char str[] = "  Hello, World  ";
+    tk_str_to_lower(str);
+
+    char* trimmed = tk_str_trim(str);
+    uint8_t contains = tk_str_contains(trimmed, "hello");
+
+    free(trimmed);
+    return contains ? 0 : 1;
+}
+
+```
+
+String utilities include counting characters/substrings, splitting by character or substring, case conversion, concatenation, trimming, and substring checks.
diff --git a/tk_strings.h b/tk_strings.h
new file mode 100644
index 0000000..f227c9f
--- /dev/null
+++ b/tk_strings.h
@@ -0,0 +1,280 @@
+#ifndef TK_STRINGS_H
+#define TK_STRINGS_H
+
+/**
+ * @file tk_strings.h
+ * @brief Single-header string utilities.
+ *
+ * Define TK_STRINGS_IMPLEMENTATION in exactly one translation unit before
+ * including this file to include the implementation.
+ */
+
+#include <stddef.h>
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @brief Count occurrences of a character in a C string.
+ * @param str Null-terminated input string. Must not be NULL.
+ * @param c Character to count.
+ * @return Number of times c appears in str.
+ */
+size_t tk_str_count_occurrences(const char* str, char c);
+
+/**
+ * @brief Count occurrences of a substring in a C string.
+ * @param str Null-terminated input string. Must not be NULL.
+ * @param substr Null-terminated substring to count. Must not be NULL or empty.
+ * @return Number of times substr appears in str.
+ */
+size_t tk_str_count_occurrences_of_substring(const char* str, const char* substr);
+
+/**
+ * @brief Split a string by a delimiter character.
+ *
+ * This copies all parts. The caller is responsible for freeing each string in
+ * result and the result array itself.
+ *
+ * @param str Null-terminated input string. Must not be NULL.
+ * @param delimiter Delimiter character.
+ * @param[out] result On success, set to a malloc'd array of char* parts.
+ * @return Number of parts. An empty string returns one empty part. If the
+ * result array allocation fails, returns 0. If a part allocation fails, returns
+ * the number of parts allocated so far and result points to that partial array.
+ */
+size_t tk_str_split_by_char(const char* str, char delimiter, char*** result);
+
+/**
+ * @brief Split a string by a delimiter substring.
+ *
+ * This copies all parts. The caller is responsible for freeing each string in
+ * result and the result array itself.
+ *
+ * @param str Null-terminated input string. Must not be NULL.
+ * @param substr Null-terminated delimiter substring. Must not be NULL or empty.
+ * @param[out] result On success, set to a malloc'd array of char* parts.
+ * @return Number of parts. If the result array allocation fails, returns 0. If
+ * a part allocation fails, returns the number of parts allocated so far and
+ * result points to that partial array.
+ */
+size_t tk_str_split_by_substring(const char* str, const char* substr, char*** result);
+
+/**
+ * @brief Convert all lowercase ASCII letters in a string to uppercase.
+ * @param str Null-terminated string to modify in place. Must not be NULL.
+ */
+void tk_str_to_upper(char* str);
+
+/**
+ * @brief Convert all uppercase ASCII letters in a string to lowercase.
+ * @param str Null-terminated string to modify in place. Must not be NULL.
+ */
+void tk_str_to_lower(char* str);
+
+/**
+ * @brief Concatenate two null-terminated strings into a newly allocated string.
+ * @param str1 First input string. Must not be NULL.
+ * @param str2 Second input string. Must not be NULL.
+ * @return Newly allocated concatenated string, or NULL on allocation error.
+ */
+char* tk_str_concat(const char* str1, const char* str2);
+
+/**
+ * @brief Trim leading and trailing whitespace from a string.
+ * @param str Null-terminated input string. Must not be NULL, empty, or all whitespace.
+ * @return Newly allocated trimmed string, or NULL on allocation error.
+ */
+char* tk_str_trim(const char* str);
+
+/**
+ * @brief Check whether a string contains a substring.
+ * @param str Null-terminated input string. Must not be NULL.
+ * @param substr Null-terminated substring to search for. Must not be NULL.
+ * @return 1 if str contains substr, 0 otherwise.
+ */
+uint8_t tk_str_contains(const char* str, const char* substr);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* TK_STRINGS_H */
+
+#ifdef TK_STRINGS_IMPLEMENTATION
+
+#include <stdbool.h>
+#include <stdlib.h>
+#include <string.h>
+
+inline size_t tk_str_count_occurrences(const char* str, const char c) {
+    size_t i = 0;
+    size_t count = 0;
+    while (str[i] != '\0') {
+        if (str[i] == c) {
+            count++;
+        }
+        i++;
+    }
+
+    return count;
+}
+
+inline size_t tk_str_count_occurrences_of_substring(const char* str, const char* substr) {
+    size_t i = 0;
+    size_t count = 0;
+    while (str[i] != '\0') {
+        if (strstr(&str[i], substr) == &str[i]) {
+            count++;
+        }
+        i++;
+    }
+    return count;
+}
+
+inline size_t tk_str_split_by_char(const char* str, const char delimiter, char*** result) {
+    const size_t size = tk_str_count_occurrences(str, delimiter);
+    char** parts = malloc((size + 1) * sizeof(char*));
+    if (parts == NULL) {
+        *result = NULL;
+        return 0;
+    }
+
+    size_t curr_part_idx = 0;
+    size_t last_part_end = 0;
+    size_t idx = 0;
+    while (str[idx] != '\0') {
+        if (str[idx] == delimiter) {
+            const size_t len = idx - last_part_end;
+            char* part = malloc(sizeof(char) * (len + 1));
+            if (part == NULL) {
+                *result = parts;
+                return curr_part_idx;
+            }
+            memcpy(part, &str[last_part_end], len);
+            part[idx - last_part_end] = '\0';
+
+            parts[curr_part_idx++] = part;
+            last_part_end = idx;
+            last_part_end++;
+        }
+        idx++;
+    }
+    char* part = malloc(sizeof(char) * (idx - last_part_end + 1));
+    if (part == NULL) {
+        *result = parts;
+        return curr_part_idx;
+    }
+    memcpy(part, &str[last_part_end], idx - last_part_end);
+    part[idx - last_part_end] = '\0';
+    parts[curr_part_idx++] = part;
+
+    *result = parts;
+    return curr_part_idx;
+}
+
+inline size_t tk_str_split_by_substring(const char* str, const char* substr, char*** result) {
+    const size_t size = tk_str_count_occurrences_of_substring(str, substr);
+    char** parts = malloc((size + 1) * sizeof(char*));
+    if (parts == NULL) {
+        *result = NULL;
+        return 0;
+    }
+
+    size_t curr_part_idx = 0;
+    size_t last_part_end = 0;
+    size_t idx = 0;
+
+    while (str[idx] != '\0') {
+        if (strstr(&str[idx], substr) == &str[idx]) {
+            const size_t len = idx - last_part_end;
+            char* part = malloc(sizeof(char) * (len + 1));
+            if (part == NULL) {
+                *result = parts;
+                return curr_part_idx;
+            }
+            memcpy(part, &str[last_part_end], len);
+            part[len] = '\0';
+
+            parts[curr_part_idx++] = part;
+            last_part_end = idx;
+            last_part_end += strlen(substr);
+        }
+        idx++;
+    }
+    const size_t len = idx - last_part_end;
+    char* part = malloc(sizeof(char) * (len + 1));
+    if (part == NULL) {
+        *result = parts;
+        return curr_part_idx;
+    }
+    memcpy(part, &str[last_part_end], len);
+    part[len] = '\0';
+
+    parts[curr_part_idx++] = part;
+
+    *result = parts;
+    return curr_part_idx;
+}
+
+inline void tk_str_to_upper(char* str) {
+    for (size_t i = 0; str[i] != '\0'; i++) {
+        if (str[i] >= 'a' && str[i] <= 'z') {
+            str[i] -= 'a' - 'A';
+        }
+    }
+}
+
+inline void tk_str_to_lower(char* str) {
+    for (size_t i = 0; str[i] != '\0'; i++) {
+        if (str[i] >= 'A' && str[i] <= 'Z') {
+            str[i] += 'a' - 'A';
+        }
+    }
+}
+
+inline char* tk_str_concat(const char* str1, const char* str2) {
+    const size_t len1 = strlen(str1);
+    const size_t len2 = strlen(str2);
+
+    char* result = malloc(sizeof(char) * (len1 + len2 + 1));
+    if (result == NULL) {
+        return NULL;
+    }
+    memcpy(result, str1, len1);
+    memcpy(&result[len1], str2, len2);
+    result[len1 + len2] = '\0';
+    return result;
+}
+
+static inline bool tk_internal_str_char_is_rubbish(const char c) {
+    return c == ' ' || c == '\t' || c == '\n' || c == '\r';
+}
+
+inline char* tk_str_trim(const char* str) {
+    size_t start = 0;
+    size_t end = strlen(str) - 1;
+    while (tk_internal_str_char_is_rubbish(str[start])) {
+        start++;
+    }
+    while (tk_internal_str_char_is_rubbish(str[end])) {
+        end--;
+    }
+
+    char* new_str = malloc(sizeof(char) * (end - start + 2));
+    if (new_str == NULL) {
+        return NULL;
+    }
+    memcpy(new_str, &str[start], end - start + 1);
+    new_str[end - start + 1] = '\0';
+
+    return new_str;
+}
+
+inline uint8_t tk_str_contains(const char* str, const char* substr) {
+    return strstr(str, substr) != NULL;
+}
+
+#endif /* TK_STRINGS_IMPLEMENTATION */