diff --git a/libc/CMakeLists.txt b/libc/CMakeLists.txt
index efba8728..9f686d94 100644
--- a/libc/CMakeLists.txt
+++ b/libc/CMakeLists.txt
@@ -7,6 +7,7 @@ set(SOURCES
     src/unistd.cpp
     src/errno.cpp
     src/string.cpp
+    src/assert.cpp
     src/atexit.cpp
     src/ctype.cpp
     src/time.cpp
diff --git a/libc/include/assert.h b/libc/include/assert.h
new file mode 100644
index 00000000..325ae3a0
--- /dev/null
+++ b/libc/include/assert.h
@@ -0,0 +1,33 @@
+/* assert.h: Debug assertions. */
+
+#ifndef _ASSERT_H
+#define _ASSERT_H
+
+#include <bits/attrs.h>
+
+#ifdef __cplusplus
+extern "C"
+{
+#endif
+
+    __noreturn void __assertion_failed(const char* file, int line, const char* function, const char* expr);
+
+#ifdef __cplusplus
+}
+#endif
+
+#if !defined(__cplusplus) && defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L
+#define static_assert _Static_assert
+#endif
+
+#ifdef NDEBUG
+#define assert(expr)
+#else
+#define assert(expr) ((expr) ? (void)0 : __assertion_failed(__FILE__, __LINE__, __func__, #expr))
+#endif
+
+#ifdef _LUNA_SOURCE
+#define release_assert(expr) ((expr) ? (void)0 : __assertion_failed(__FILE__, __LINE__, __func__, #expr))
+#endif
+
+#endif
diff --git a/libc/include/bits/attrs.h b/libc/include/bits/attrs.h
index c9f6cf58..90de54d0 100644
--- a/libc/include/bits/attrs.h
+++ b/libc/include/bits/attrs.h
@@ -3,7 +3,7 @@
 #ifndef _BITS_ATTRS_H
 #define _BITS_ATTRS_H
 
-#if !defined(_STDLIB_H) && !defined(_STRING_H)
+#if !defined(_STDLIB_H) && !defined(_STRING_H) && !defined(_ASSERT_H)
 #error "Never include bits/attrs.h directly; use one of the standard library headers."
 #endif
 
diff --git a/libc/src/assert.cpp b/libc/src/assert.cpp
new file mode 100644
index 00000000..f24110b6
--- /dev/null
+++ b/libc/src/assert.cpp
@@ -0,0 +1,14 @@
+#include <assert.h>
+#include <luna/Attributes.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+extern "C"
+{
+    _weak [[noreturn]] void __assertion_failed(const char* file, int line, const char* function, const char* expr)
+    {
+        // FIXME: Output to standard error instead of standard output.
+        printf("%s:%d: %s: Assertion '%s' failed.\n", file, line, function, expr);
+        abort();
+    }
+}
diff --git a/luna/include/luna/SystemError.h b/luna/include/luna/SystemError.h
index 45f4f921..9985897a 100644
--- a/luna/include/luna/SystemError.h
+++ b/luna/include/luna/SystemError.h
@@ -54,10 +54,12 @@
 #define ETIMEDOUT 110   // Connection timed out
 #define EALREADY 114    // Operation already in progress
 
-#if defined(IN_MOON) || defined(USE_FREESTANDING) || defined(_LUNA_SYSTEM_ERROR_EXTENSIONS)
+#if defined(IN_MOON) || defined(USE_FREESTANDING) || defined(_LUNA_SYSTEM_ERROR_EXTENSIONS) || defined(_LUNA_SOURCE)
 // This one is Luna-specific.
 #define EFIXME 342 // Functionality not yet implemented
 
+#ifndef _LUNA_SOURCE
 const char* error_string(int error);
+#endif
 
 #endif