diff --git a/README.md b/README.md
index cec3d39..a6262ab 100644
--- a/README.md
+++ b/README.md
@@ -27,6 +27,21 @@ Right now, the following configurations exist:
 
 - `foundation_libc_panic_handler`, which allows users to catch detectable undefined behaviour.
 
+You can also configure the libc by chosing the build mode:
+
+- `Debug`: Implements additional safety checks and adds breakpoints in panics.
+- `ReleaseSafe`: Keeps the safety checks, but removes breakpoints.
+- `ReleaseSmall`: Still keeps a certain amount of safety, but drops long internal strings to reduce code and ram size.
+- `ReleaseFast`: Gotta go fast. Drops all safety and assumes all code behaves well.
+
+There are also certain "usage" configurations that can be chosen to affect behaviour when *using* the headers. Those are implemented as C macros/defines:
+
+- `FOUNDATION_LIBC_ASSERT` is a global macro that defines how `assert()` should behave:
+  - `FOUNDATION_LIBC_ASSERT_DEFAULT=0`: Behaves like a regular assert that can print file name, assertion message and line.
+  - `FOUNDATION_LIBC_ASSERT_NOFILE=1`: Drops the filename from the assertion to reduce code size.
+  - `FOUNDATION_LIBC_ASSERT_NOMSG=2`: Additionally drops the assertion message from the assertion to reduce code size.
+  - `FOUNDATION_LIBC_ASSERT_EXPECTED=3`: Replaces `assert(…)` with a construct that tells the compiler the assertion is always met. Makes code very fast. Assertions aren't checked.
+
 ## Development
 
 Zig Version: 0.11
@@ -75,7 +90,7 @@ Which functions belong into which header can be figured out by taking a look at
 
 | Header File     | Header Status | Implementation Status | Description                                                                                             |
 | --------------- | ------------- | --------------------- | ------------------------------------------------------------------------------------------------------- |
-| `assert.h`      | ❌             |                       | Conditionally compiled macro that compares its argument to zero                                         |
+| `assert.h`      | ✅             | ✅                     | Conditionally compiled macro that compares its argument to zero                                         |
 | `complex.h`     | ❌             |                       | (since C99) Complex number arithmetic                                                                   |
 | `ctype.h`       | ✅             | ✅                     | Functions to determine the type contained in character data                                             |
 | `errno.h`       | ✅             | ✅                     | Macros reporting error conditions                                                                       |
diff --git a/build.zig b/build.zig
index b1df2a9..f73f285 100644
--- a/build.zig
+++ b/build.zig
@@ -112,6 +112,32 @@ pub fn build(b: *std.Build) void {
 
             validation_step.dependOn(&ext_compiler.step);
         }
+
+        // Validate all modes of assertion:
+        for ([_][]const u8{
+            "FOUNDATION_LIBC_ASSERT_DEFAULT",
+            "FOUNDATION_LIBC_ASSERT_NOFILE",
+            "FOUNDATION_LIBC_ASSERT_NOMSG",
+            "FOUNDATION_LIBC_ASSERT_EXPECTED",
+        }) |assert_mode| {
+            // Check if the syntax of all of our header files is valid:
+            const assert_validator = b.addStaticLibrary(.{
+                .name = "assert-validator",
+                .target = .{}, // just build for the host
+                .optimize = .Debug,
+            });
+            assert_validator.addCSourceFile(.{
+                .file = .{ .path = "tests/assert-validator.c" },
+                .flags = &common_c_flags,
+            });
+            assert_validator.addIncludePath(include_path);
+            _ = assert_validator.getEmittedBin();
+
+            assert_validator.defineCMacro("FOUNDATION_LIBC_ASSERT", assert_mode);
+
+            // Just compile, do not install:
+            validation_step.dependOn(&assert_validator.step);
+        }
     }
 }
 
diff --git a/include/assert.h b/include/assert.h
index 81cbd9e..16c4979 100644
--- a/include/assert.h
+++ b/include/assert.h
@@ -1,16 +1,43 @@
 #ifndef _FOUNDATION_LIBC_ASSERT_H_
 #define _FOUNDATION_LIBC_ASSERT_H_
 
-#ifndef NDEBUG
-#define assert(expr)
-#else
-extern void __assert(char const * assertion, char const * file, unsigned line) __attribute__((__noreturn__));
+#include "foundation/builtins.h"
+
+extern FOUNDATION_LIBC_NORETURN void foundation_libc_assert(char const * assertion, char const * file, unsigned line);
+
+#if FOUNDATION_LIBC_ASSERT == FOUNDATION_LIBC_ASSERT_DEFAULT
+
+#define assert(expr)                                           \
+    do {                                                       \
+        if ((expr) == 0) {                                     \
+            foundation_libc_assert(#expr, __FILE__, __LINE__); \
+        }                                                      \
+    } while (0)
+
+#elif FOUNDATION_LIBC_ASSERT == FOUNDATION_LIBC_ASSERT_NOFILE
 
-#define assert(expr) \
-    ((expr)          \
-         ? void(0)   \
-         : __assert(#expr, __FILE__, __LINE__))
+#define assert(expr)                                     \
+    do {                                                 \
+        if ((expr) == 0) {                               \
+            foundation_libc_assert(#expr, "", __LINE__); \
+        }                                                \
+    } while (0)
 
+#elif FOUNDATION_LIBC_ASSERT == FOUNDATION_LIBC_ASSERT_NOMSG
+
+#define assert(expr)                                  \
+    do {                                              \
+        if ((expr) == 0) {                            \
+            foundation_libc_assert("", "", __LINE__); \
+        }                                             \
+    } while (0)
+
+#elif FOUNDATION_LIBC_ASSERT == FOUNDATION_LIBC_ASSERT_EXPECTED
+
+#define assert(expr) FOUNDATION_LIBC_EXPECT(expr)
+
+#else
+#error "bad definition of FOUNDATION_LIBC_ASSERT_DEFAULT!"
 #endif
 
 #endif
diff --git a/include/foundation/builtins.h b/include/foundation/builtins.h
new file mode 100644
index 0000000..e95803e
--- /dev/null
+++ b/include/foundation/builtins.h
@@ -0,0 +1,37 @@
+#ifndef _FOUNDATION_LIBC_BUILTINS_H_
+#define _FOUNDATION_LIBC_BUILTINS_H_
+
+#include <stddef.h>
+
+#define FOUNDATION_LIBC_ASSERT_DEFAULT 0
+#define FOUNDATION_LIBC_ASSERT_NOFILE 1
+#define FOUNDATION_LIBC_ASSERT_NOMSG 2
+#define FOUNDATION_LIBC_ASSERT_EXPECTED 3
+
+#ifndef FOUNDATION_LIBC_ASSERT
+#define FOUNDATION_LIBC_ASSERT FOUNDATION_LIBC_ASSERT_DEFAULT
+#endif
+
+#if defined(__clang__)
+
+#define FOUNDATION_LIBC_NORETURN __attribute__((__noreturn__))
+#define FOUNDATION_LIBC_EXPECT(expr) (__builtin_expect(!(expr), 0))
+
+#elif defined(__GNUC__) || defined(__GNUG__)
+
+#define FOUNDATION_LIBC_NORETURN __attribute__((__noreturn__))
+#define FOUNDATION_LIBC_EXPECT(expr) (__builtin_expect(!(expr), 0))
+
+#elif defined(_MSC_VER)
+
+#define FOUNDATION_LIBC_NORETURN __declspec(noreturn)
+#define FOUNDATION_LIBC_EXPECT(expr) (__assume((expr)))
+
+#else
+
+#define FOUNDATION_LIBC_NORETURN
+#define FOUNDATION_LIBC_EXPECT(expr)
+
+#endif
+
+#endif
diff --git a/src/modules/assert.zig b/src/modules/assert.zig
index fc181de..436760a 100644
--- a/src/modules/assert.zig
+++ b/src/modules/assert.zig
@@ -1,14 +1,20 @@
 const std = @import("std");
+const builtin = @import("builtin");
 
-export fn __assert(
+export fn foundation_libc_assert(
     assertion: ?[*:0]const u8,
     file: ?[*:0]const u8,
     line: c_uint,
 ) noreturn {
-    var buf: [256]u8 = undefined;
-    const str = std.fmt.bufPrint(&buf, "assertion failed: '{?s}' in file {?s} line {}", .{ assertion, file, line }) catch {
-        @panic("assertion failed");
-    };
-
-    @panic(str);
+    switch (builtin.mode) {
+        .Debug, .ReleaseSafe => {
+            var buf: [256]u8 = undefined;
+            const str = std.fmt.bufPrint(&buf, "assertion failed: '{?s}' in file {?s} line {}", .{ assertion, file, line }) catch {
+                @panic("assertion failed");
+            };
+            @panic(str);
+        },
+        .ReleaseSmall => @panic("assertion failed"),
+        .ReleaseFast => unreachable,
+    }
 }
diff --git a/tests/assert-validator.c b/tests/assert-validator.c
new file mode 100644
index 0000000..b75ca55
--- /dev/null
+++ b/tests/assert-validator.c
@@ -0,0 +1,25 @@
+#include <assert.h>
+
+#ifndef FOUNDATION_LIBC_ASSERT
+#error "FOUNDATION_LIBC_ASSERT wasn't implicitly or explicitly defined by assert.h"
+#endif
+
+#pragma GCC diagnostic ignored "-Wunused-function"
+#pragma GCC diagnostic ignored "-Wmissing-prototypes"
+//
+
+void assert_dynamic(int ok) {
+    assert(ok);
+}
+
+void assert_ok(void) {
+    assert(1);
+}
+
+// suppress noreturn diagnostic for missing "noreturn"
+#pragma GCC diagnostic ignored "-Wmissing-noreturn"
+//
+
+void assert_bad(void) {
+    assert(0);
+}
diff --git a/tests/syntactic-validation.c b/tests/syntactic-validation.c
index c31050e..375c489 100644
--- a/tests/syntactic-validation.c
+++ b/tests/syntactic-validation.c
@@ -1,5 +1,6 @@
 
 // our own files must be included as non-system includes to trigger warnings:
+#include "assert.h"
 #include "ctype.h"
 #include "errno.h"
 #include "foundation/libc.h"