You can use C but you are not forced to. In fact, you can write C and convert it automatically to D (though it will need some amount of manual editing afterwards). C is supported as a syntax option but it's still the same compiler for both under the hood. As rightly pointed out by the user above, you can write the same high performance code in the D syntax. The reverse is not true, though -- using high level concepts like classes and GC allocation is not supported in the C syntax.