Functional communication training (FCT) is a frequently used treatment for reducing problem behavior exhibited by individuals with developmental disabilities. Once the operant function of problem behavior is identified by a functional analysis, the client is taught to emit an appropriate communicative response to obtain the reinforcer that is responsible for behavioral maintenance. Studies on FCT have typically used small numbers of participants, have reported primarily on clients for whom FCT was successful, and have varied with respect to their use of other treatment components. The main purposes of the present study were to evaluate the efficacy of FCT for treating severe problem behavior in a relatively large sample of individuals with mental retardation (N = 21) and to determine the contribution of extinction and punishment components to FCT treatment packages. FCT with extinction was effective in reducing problem behavior for the majority of clients and resulted in at least a 90% reduction in problem behavior in nearly half the applications. However, when demand or delay-to-reinforcement fading was added to FCT with extinction, treatment efficacy was reduced in about one half of the applications. FCT with punishment (both with and without fading) resulted in at least a 90% reduction in problem behavior for every case in which it was applied.