JEFF is an explanation facility framework written in Java. Explanation facilities date from the era of expert systems (ES), where they were used in order to provide an explanation about the inference process. The explanation they provided was supposed to clarify how the ES reached its conclusions or why it asked some question during fact acquisition. Nowadays, traditional ES development environments ("shells") have been replaced by rule engines (RE) and business rule management systems (BRMS), which seem to lack explanation facility functionality. JEFF was created in order to remedy this.