FitNesse started off as a a fancy wrapper for FIT. FIT originally written in Java by WardCunningham[?], is the engine that actually runs your tests. There is an additional component called FitServer[?] that allows FitNesse to interface with FIT. FitNesse is not limited to running only Java tests. Given an implementation of FIT and the corresponding FitServer[?], FitNesse can execute tests in any language. Below is a list of all languages with FIT/FitServer implementations.

Note: Slim is nowadays the preferred test system for FitNesse.