.NET 4 most certainly does optimize tail-calls, and the 64-bit versions of the CLR contain additional optimizations that the 32-bit versions don't. In fact, F#

.NET 4 most certainly does optimize tail-calls, and the 64-bit versions of the CLR contain additional optimizations that the 32-bit versions don't. In fact, F# is able to target previous version of the CLR (.NET 2.0, 3.0, and 3.5) and I know for a fact that they also support the 'tail.' opcode and perform some TCO (though it handles fewer edge-cases than .NET 4.0 and 4.5).