diff --git a/tests/test_internal.c b/tests/test_internal.c
new file mode 100644
index 0000000..3ed8b66
--- /dev/null
+++ b/tests/test_internal.c
@@ -0,0 +1,48 @@
+#include "utest_lib.h"
+
+#include "../src/internal/byte_swap.h"
+#include "../src/internal/byte_codec.h"
+
+const char* test_internal__byte_swap()
+{
+  UTEST_ASSERT(foug_uint16_bswap(0x1122) == 0x2211);
+  UTEST_ASSERT(foug_uint32_bswap(0x11223344) == 0x44332211);
+  return NULL;
+}
+
+const char* test_internal__byte_codec()
+{
+  { /* decode */
+    const uint8_t data[] = { 0x11, 0x22, 0x33, 0x44 };
+    UTEST_ASSERT(foug_decode_uint16_le(data) == 0x2211);
+    UTEST_ASSERT(foug_decode_uint16_be(data) == 0x1122);
+    UTEST_ASSERT(foug_decode_uint32_le(data) == 0x44332211);
+    UTEST_ASSERT(foug_decode_uint32_be(data) == 0x11223344);
+  }
+
+  { /* encode */
+    uint8_t data[] = { 0, 0, 0, 0 };
+    foug_encode_uint16_le(0x1122, data);
+    UTEST_ASSERT(data[0] == 0x22 && data[1] == 0x11);
+    foug_encode_uint32_le(0x11223344, data);
+    UTEST_ASSERT(data[0] == 0x44 && data[1] == 0x33 && data[2] == 0x22 && data[3] == 0x11);
+    foug_encode_uint32_be(0x11223344, data);
+    UTEST_ASSERT(data[3] == 0x44 && data[2] == 0x33 && data[1] == 0x22 && data[0] == 0x11);
+  }
+
+  return NULL;
+}
+
+const char* test_internal__ascii_parse()
+{
+}
+
+const char* all_tests()
+{
+  UTEST_SUITE_START();
+  UTEST_RUN(test_internal__byte_swap);
+  UTEST_RUN(test_internal__byte_codec);
+  return NULL;
+}
+
+UTEST_MAIN(all_tests)
\ No newline at end of file
diff --git a/tests/utest_lib.h b/tests/utest_lib.h
new file mode 100644
index 0000000..bccb412
--- /dev/null
+++ b/tests/utest_lib.h
@@ -0,0 +1,38 @@
+#ifndef UTEST_LIB_H
+#define UTEST_LIB_H
+
+#include <stdio.h>
+#include <stdlib.h>
+
+#define UTEST_SUITE_START() const char *message = NULL
+
+#define UTEST_ASSERT(test) if (!(test))\
+                           { printf("  ERROR : %s   (line = %i, file = %s)", #test, __LINE__, __FILE__);\
+                             return #test; }
+
+#define UTEST_ASSERT_MSG(test, message) if (!(test))\
+                                        { printf(message); return message; }
+                                        
+#define UTEST_RUN(test) printf("\n-----%s", " " #test); \
+                        message = test();\
+                        tests_run++;\
+                        if (message) return message;
+
+#define UTEST_MAIN(name) int main(int argc, char *argv[]) {\
+                           argc = 1;\
+                           printf("----\nRUNNING: %s\n", argv[0]);\
+                           const char *result = name();\
+                           if (result != 0) {\
+                             printf("\n\nFAILED: %s\n", result);\
+                           }\
+                           else {\
+                             printf("\n\nALL TESTS PASSED\n");\
+                           }\
+                           printf("Tests run: %d\n", tests_run);\
+                           exit(result != 0);\
+                         }
+
+
+static int tests_run;
+
+#endif /* UTEST_LIB_H */