diff --git a/src/libcore/f32.rs b/src/libcore/f32.rs
index 6a053efb3db..89a8bfa424f 100644
--- a/src/libcore/f32.rs
+++ b/src/libcore/f32.rs
@@ -4,6 +4,7 @@
 
 import cmath::c_float::*;
 import cmath::c_float_targ_consts::*;
+import num::num;
 
 // FIXME find out why these have to be exported explicitly
 
@@ -19,6 +20,7 @@ export mul_add, fmax, fmin, nextafter, frexp, hypot, ldexp;
 export lgamma, ln, log_radix, ln1p, log10, log2, ilog_radix;
 export modf, pow, round, sin, sinh, sqrt, tan, tanh, tgamma, trunc;
 export signbit;
+export num;
 
 // These are not defined inside consts:: for consistency with
 // the integer types
@@ -176,6 +178,17 @@ pure fn log2(n: f32) -> f32 {
     ret ln(n) / consts::ln_2;
 }
 
+impl num of num for f32 {
+    fn add(&&other: f32)    -> f32 { ret self + other; }
+    fn sub(&&other: f32)    -> f32 { ret self - other; }
+    fn mul(&&other: f32)    -> f32 { ret self * other; }
+    fn div(&&other: f32)    -> f32 { ret self / other; }
+    fn modulo(&&other: f32) -> f32 { ret self % other; }
+
+    fn to_int()         -> int { ret self as int; }
+    fn from_int(n: int) -> f32 { ret n as f32;    }
+}
+
 //
 // Local Variables:
 // mode: rust
diff --git a/src/libcore/f64.rs b/src/libcore/f64.rs
index e0a10bc7889..151b90d4fcb 100644
--- a/src/libcore/f64.rs
+++ b/src/libcore/f64.rs
@@ -4,6 +4,7 @@
 
 import cmath::c_double::*;
 import cmath::c_double_targ_consts::*;
+import num::num;
 
 // Even though this module exports everything defined in it,
 // because it contains re-exports, we also have to explicitly
@@ -21,6 +22,7 @@ export lgamma, ln, log_radix, ln1p, log10, log2, ilog_radix;
 export modf, pow, round, sin, sinh, sqrt, tan, tanh, tgamma, trunc;
 export signbit;
 export epsilon;
+export num;
 
 // These are not defined inside consts:: for consistency with
 // the integer types
@@ -197,6 +199,17 @@ pure fn log2(n: f64) -> f64 {
     ret ln(n) / consts::ln_2;
 }
 
+impl num of num for f64 {
+    fn add(&&other: f64)    -> f64 { ret self + other; }
+    fn sub(&&other: f64)    -> f64 { ret self - other; }
+    fn mul(&&other: f64)    -> f64 { ret self * other; }
+    fn div(&&other: f64)    -> f64 { ret self / other; }
+    fn modulo(&&other: f64) -> f64 { ret self % other; }
+
+    fn to_int()         -> int { ret self as int; }
+    fn from_int(n: int) -> f64 { ret n as f64;    }
+}
+
 //
 // Local Variables:
 // mode: rust
diff --git a/src/libcore/float.rs b/src/libcore/float.rs
index e922737a815..a2b5197277a 100644
--- a/src/libcore/float.rs
+++ b/src/libcore/float.rs
@@ -17,6 +17,7 @@ export lgamma, ln, log_radix, ln1p, log10, log2, ilog_radix;
 export modf, pow, round, sin, sinh, sqrt, tan, tanh, tgamma, trunc;
 export signbit;
 export pow_with_uint;
+export num;
 
 // export when m_float == c_double
 
@@ -26,6 +27,7 @@ export j0, j1, jn, y0, y1, yn;
 
 import m_float = f64;
 import f64::*;
+import num::num;
 
 const NaN: float = 0.0/0.0;
 
@@ -408,6 +410,17 @@ fn sin(x: float) -> float { f64::sin(x as f64) as float }
 fn cos(x: float) -> float { f64::cos(x as f64) as float }
 fn tan(x: float) -> float { f64::tan(x as f64) as float }
 
+impl num of num for float {
+    fn add(&&other: float)    -> float { ret self + other; }
+    fn sub(&&other: float)    -> float { ret self - other; }
+    fn mul(&&other: float)    -> float { ret self * other; }
+    fn div(&&other: float)    -> float { ret self / other; }
+    fn modulo(&&other: float) -> float { ret self % other; }
+
+    fn to_int()         -> int   { ret self as int; }
+    fn from_int(n: int) -> float { ret n as float;  }
+}
+
 #[test]
 fn test_from_str() {
    assert from_str("3") == some(3.);
@@ -501,6 +514,25 @@ fn test_to_str_inf() {
     assert to_str(-infinity, 10u) == "-inf";
 }
 
+#[test]
+fn test_ifaces() {
+    fn test<U:num>(ten: U) {
+        assert (ten.to_int() == 10);
+
+        let two = ten.from_int(2);
+        assert (two.to_int() == 2);
+
+        assert (ten.add(two) == ten.from_int(12));
+        assert (ten.sub(two) == ten.from_int(8));
+        assert (ten.mul(two) == ten.from_int(20));
+        assert (ten.div(two) == ten.from_int(5));
+        assert (ten.modulo(two) == ten.from_int(0));
+    }
+
+    test(10.0);
+}
+
+
 //
 // Local Variables:
 // mode: rust
diff --git a/src/libcore/num.rs b/src/libcore/num.rs
new file mode 100644
index 00000000000..7d57f7ff55b
--- /dev/null
+++ b/src/libcore/num.rs
@@ -0,0 +1,15 @@
+#[doc="An interface for numbers."]
+
+iface num {
+    // FIXME: Cross-crate overloading doesn't work yet.
+    // FIXME: Interface inheritance.
+    fn add(&&other: self) -> self;
+    fn sub(&&other: self) -> self;
+    fn mul(&&other: self) -> self;
+    fn div(&&other: self) -> self;
+    fn modulo(&&other: self) -> self;
+
+    fn to_int() -> int;
+    fn from_int(n: int) -> self;    // TODO: Static functions.
+}
+