diff --git a/array.rs b/array.rs
index d8d431b..ac1f45a 100644
--- a/array.rs
+++ b/array.rs
@@ -80,7 +80,7 @@ impl<ElemRefType:AbstractCFTypeRef> Index<uint,ElemRefType> for CFArray<ElemRefT
     // Generally, when array elements are Core Foundation objects (not
     // always true), they need to be wrapped with CFWrapper::wrap_shared.
     fn index(&self, idx: uint) -> ElemRefType {
-        fail_unless!(idx < self.len());
+        assert!(idx < self.len());
         unsafe { 
             let elem = CFArrayGetValueAtIndex(*self.borrow_ref(), idx as CFIndex);
             // Don't return a wrapped thing, since we don't know whether
@@ -136,11 +136,11 @@ fn should_box_and_unbox() {
         sum += CFWrapper::wrap_shared(*elem).to_i32();
     }
 
-    fail_unless!(sum == 15);
+    assert!(sum == 15);
 
     for arr.each |elem: &CFNumberRef| {
         sum += CFWrapper::wrap_shared(*elem).to_i32();
     }
 
-    fail_unless!(sum == 30);
+    assert!(sum == 30);
 }
diff --git a/base.rs b/base.rs
index 7ea7fef..e832d1a 100644
--- a/base.rs
+++ b/base.rs
@@ -42,7 +42,7 @@ impl AbstractCFTypeRef for CFTypeRef {
 
 pub fn downcast<T:AbstractCFTypeRef>(r: CFTypeRef) -> T {
     unsafe {
-        fail_unless!(CFGetTypeID(r) == AbstractCFTypeRef::type_id::<T>());
+        assert!(CFGetTypeID(r) == AbstractCFTypeRef::type_id::<T>());
         cast::transmute::<CFTypeRef, T>(r)
     }
 }
@@ -63,7 +63,7 @@ impl<T,E1,E2> Drop for CFWrapper<T,E1,E2> {
             // cannot make virtual method calls using trait
             // types. Instead, just transmute the bugger.
             let this: &RawCFWrapper = cast::transmute::<&CFWrapper<T,E1,E2>, &RawCFWrapper>(self);
-            fail_unless!(CFGetRetainCount(this.obj) > 0 as CFIndex);
+            assert!(CFGetRetainCount(this.obj) > 0 as CFIndex);
             CFRelease(this.obj)
         }
     }
@@ -111,7 +111,7 @@ pub impl<T:Copy + AbstractCFTypeRef, E1, E2> CFWrapper<T,E1,E2> {
 
     fn from_CFType(wrapper: CFType) -> CFWrapper<T,E1,E2> {
         unsafe {
-            fail_unless!(wrapper.type_id() == AbstractCFTypeRef::type_id::<T>());
+            assert!(wrapper.type_id() == AbstractCFTypeRef::type_id::<T>());
             cast::transmute::<CFType,CFWrapper<T,E1,E2>>(wrapper)
         }
     }
diff --git a/dictionary.rs b/dictionary.rs
index c8bf996..9d11e53 100644
--- a/dictionary.rs
+++ b/dictionary.rs
@@ -69,7 +69,7 @@ pub impl<KeyRefType: Copy + AbstractCFTypeRef, ValueRefType: Copy + AbstractCFTy
             }
         }
 
-        fail_unless!(keys.len() == values.len());
+        assert!(keys.len() == values.len());
 
         let dictionary_ref : CFDictionaryRef;
         unsafe {
diff --git a/number.rs b/number.rs
index f9ec159..f61c716 100644
--- a/number.rs
+++ b/number.rs
@@ -59,7 +59,7 @@ pub impl CFNumber {
 
     fn to_i8(&self) -> i8 {
         let ty = kCFNumberSInt8Type;
-        fail_unless!(self.has_number_type(ty));
+        assert!(self.has_number_type(ty));
         unsafe {
             let val: i8 = 0i8;
             if !CFNumberGetValue(self.obj, ty, cast::transmute::<&i8, *c_void>(&val)) {
@@ -71,7 +71,7 @@ pub impl CFNumber {
 
     fn to_i16(&self) -> i16 {
         let ty = kCFNumberSInt16Type;
-        fail_unless!(self.has_number_type(ty));
+        assert!(self.has_number_type(ty));
         unsafe {
             let val: i16 = 0i16;
             if !CFNumberGetValue(self.obj, ty, cast::transmute::<&i16, *c_void>(&val)) {
@@ -83,7 +83,7 @@ pub impl CFNumber {
 
     fn to_i32(&self) -> i32 {
         let ty = kCFNumberSInt32Type;
-        fail_unless!(self.has_number_type(ty));
+        assert!(self.has_number_type(ty));
         unsafe {
             let val: i32 = 0i32;
             if !CFNumberGetValue(self.obj, ty, cast::transmute::<&i32, *c_void>(&val)) {
@@ -95,7 +95,7 @@ pub impl CFNumber {
 
     fn to_float(&self) -> float {
         unsafe {
-            fail_unless!(self.has_float_type());
+            assert!(self.has_float_type());
             let ty = CFNumberGetType(self.obj);
             if ty == kCFNumberFloat32Type || ty == kCFNumberFloatType {
                 let mut val: libc::c_float = 0.0f as libc::c_float;
diff --git a/string.rs b/string.rs
index 10f1ed3..3c59e22 100644
--- a/string.rs
+++ b/string.rs
@@ -283,10 +283,10 @@ impl ToStr for CFString {
                                                  buffer.len() as CFIndex,
                                                  ptr::to_unsafe_ptr(&bytes_used)) as uint;
 
-            fail_unless!(chars_written == char_len);
+            assert!(chars_written == char_len);
             // this is dangerous; we over-allocate and nul-terminate the string (during
             // initialization)
-            fail_unless!(bytes_used + 1 == buffer.len() as CFIndex);
+            assert!(bytes_used + 1 == buffer.len() as CFIndex);
             // then, reinterpret it as as string. you have been warned!
             let casted_str : ~str = cast::transmute::<~[u8], ~str>(buffer);
             // sanity check.
@@ -414,5 +414,5 @@ fn string_and_back() {
     let original = "The quick brown fox jumped over the slow lazy dog.";
     let cfstr = CFString::new_static(original);
     let converted = cfstr.to_str();
-    fail_unless!(original == converted);
+    assert!(original == converted);
 }