Research into psychosocial interventions (particularly cognitive-behavior therapies and social skills training) for social-communication deficits among individuals with autism spectrum disorder (ASD) has proliferated over the past decade. While this research has provided some empirical support for the efficacy of these interventions, little work has begun to elucidate therapeutic mechanisms-the when, why, how, for whom, and under what conditions an intervention may produce change, identification of mechanisms underlying these effects should help advance ASD intervention research. This article describes methods for assessing such mechanisms (ie, mediators and moderators) and presents promising candidates for common mechanisms impacting treatment response: behavior modification, therapeutic relationship, social knowledge, social motivation, social information processing, executive functioning, and internalizing comorbidities. Finally, future directions are discussed as a program of psychosocial intervention research designed to identify predictors of individual differences in treatment response (including biomarkers), isolate active therapeutic ingredients, and promote dissemination of optimized interventions.