diff --git a/makefile b/makefile
index a20af44..514505e 100644
--- a/makefile
+++ b/makefile
@@ -29,14 +29,20 @@ program_obj_files = stack.o bintree.o numbers.o timer.o highscore.o
 doble : main.o $(program_obj_files)
 	$(CC) $(FLAGS) $^ -o doble
 
-$(program_obj_filesobj_files): %.o: %.c
+$(program_obj_files): %.o: %.c
 	$(CC) -c $(FLAGS) $^ -o $@
 
 # --------------------------
 # Unit Tests
 # --------------------------
-unitTests:
-	echo "needs to be implemented"
+
+TEST_BIN = runTests
+
+unitTests: stack.o test_stack.o
+	$(CC) $(FLAGS) -I$(unityfolder) -o $(TEST_BIN) stack.o test_stack.o $(unityfolder)/unity.c
+
+test_stack.o: test_stack.c
+	$(CC) $(FLAGS) -I$(unityfolder) -c test_stack.c -o test_stack.o
 
 # --------------------------
 # Clean
diff --git a/stack.c b/stack.c
index e3a90d4..35e7708 100644
--- a/stack.c
+++ b/stack.c
@@ -1,33 +1,82 @@
-#include <stdlib.h>
 #include "stack.h"
+#include <stdio.h>
+#include <stdlib.h>
 
-//TODO: grundlegende Stackfunktionen implementieren:
-/*  * `push`: legt ein Element oben auf den Stack,
-    * `pop`: entfernt das oberste Element,
-    * `top`: liefert das oberste Element zurück,
-    * `clearStack`: gibt den gesamten Speicher frei. */
+/*typedef struct {
 
-// Pushes data as pointer onto the stack.
-StackNode *push(StackNode *stack, void *data)
-{
+  void *data;
+  struct StackNode *next;
 
+} StackNode;*/
+
+// TODO: grundlegende Stackfunktionen implementieren:
+/* `push`: legt ein Element oben auf den Stack,
+ * `pop`: entfernt das oberste Element,
+ * `top`: liefert das oberste Element zurück,
+ * `clearStack`: gibt den gesamten Speicher frei. */
+
+// [A] -> [B] -> [C] -> NULL
+// head -> stack.next
+
+StackNode *createNode(void *data) {
+
+  StackNode *node =
+      malloc(sizeof(StackNode)); // Speicher reservieren, Speicherplatz für das
+                                 // struct StackNode
+
+  if (node == NULL)
+    return NULL; // Speicher konnte nicht reserviert werden
+
+  node->data = data; // Zeiger auf data neuer node
+  node->next = NULL; // nächster Zeiger ist NULL, Ende der Liste
+
+  return node; // pointer auf den neuen Knoten zurückgeben
 }
 
-// Deletes the top element of the stack (latest added element) and releases its memory. (Pointer to data has to be
-// freed by caller.)
-StackNode *pop(StackNode *stack)
-{
+// Pushes data as pointer onto the stack.
+StackNode *push(StackNode *stack, void *data) {
 
+  StackNode *newNode = createNode(data); // Speicher für neuen Knoten
+                                         // reservieren
+
+  if (newNode == NULL) { // wenn Speicher nicht reserviert werden konnte, wird
+                         // stack unverändert zurückgegeben
+    return stack;
+  }
+
+  newNode->next = stack; // pointer verschieben
+
+  return newNode; // Zeiger auf neuen Speicherbereich zurückgeben
+}
+
+// Deletes the top element of the stack (latest added element) and releases its
+// memory. (Pointer to data has to be freed by caller.)
+StackNode *pop(StackNode *stack) {
+
+  if (stack == NULL)
+    return NULL;
+
+  StackNode *nextNode = stack->next;
+
+  free(stack);
+
+  stack = NULL;
+
+  return nextNode;
 }
 
 // Returns the data of the top element.
-void *top(StackNode *stack)
-{
-
-}
+void *top(StackNode *stack) { return stack != NULL ? stack->data : NULL; }
 
 // Clears stack and releases all memory.
-void clearStack(StackNode *stack)
-{
+StackNode *clearStack(StackNode *stack) {
 
+  while (stack != NULL) {
+
+    StackNode *next = stack->next;
+    free(stack);
+    stack = next;
+  }
+
+  return NULL;
 }
\ No newline at end of file
diff --git a/stack.h b/stack.h
index f7d542d..f90d217 100644
--- a/stack.h
+++ b/stack.h
@@ -1,25 +1,34 @@
 #ifndef STACK_H
 #define STACK_H
 
-/* A stack is a special type of queue which uses the LIFO (last in, first out) principle. 
-This means that with each new element all other elements are pushed deeper into the stack. 
-The latest element is taken from the stack. */
+/* A stack is a special type of queue which uses the LIFO (last in, first out)
+principle. This means that with each new element all other elements are pushed
+deeper into the stack. The latest element is taken from the stack. */
 
 #include <stdlib.h>
 
-//TODO: passenden Datentyp als struct anlegen
+// TODO: passenden Datentyp als struct anlegen
+
+typedef struct StackNode {
+
+  void *data;
+  struct StackNode *next;
+
+} StackNode;
+
+StackNode *createNode(void *data);
 
 // Pushes data as pointer onto the stack.
 StackNode *push(StackNode *stack, void *data);
 
-// Deletes the top element of the stack (latest added element) and releases its memory. (Pointer to data has to be
-// freed by caller.)
+// Deletes the top element of the stack (latest added element) and releases its
+// memory. (Pointer to data has to be freed by caller.)
 StackNode *pop(StackNode *stack);
 
 // Returns the data of the top element.
 void *top(StackNode *stack);
 
 // Clears stack and releases all memory.
-void clearStack(StackNode *stack);
+StackNode *clearStack(StackNode *stack);
 
 #endif