The process of functional assessment has emerged as an essential component for intervention development. Applications across divergent types of problem behavior, however, remain limited. This study evaluated the applicability of this promising approach to students with selective mutism. Two middle school students served as participants. The functional assessment included indirect and direct methods as well as a specially designed student interview that did not require speech. Individualized interventions were developed and experimentally evaluated. Results indicated that the assessment-based interventions effectively increased speaking in school contexts. (Contains 2 figures.)