Despite advances in early identification of and intervention for children with autism spectrum disorders (ASD), the long-term outcomes for children with ASD remain variable. As many as 40% of children with ASD are minimally verbal at 9 years of age, and 75% of adults with ASD have persistent social communication (SC) difficulties. Furthermore, as many as 70% of children with ASD have a co-occurring diagnosis of disruptive behavior (DB) disorder. Parents play an important role in SC development and in the prevention of and intervention for DB. As such, the overarching goal of the proposed study is to: (a) determine how best to sequence two parent-mediated interventions: an SC intervention (Project ImPACT, Improving Parents as Communication Teachers) and a DB intervention (Parent Training for Disruptive Behavior) and (b) examine moderators and mediators of intervention outcomes. While evidence of efficacy and feasibility exist for both of these interventions individually, an adaptive intervention approach that considers and optimizes both interventions has not been evaluated. This type of adaptive intervention approach may be particularly needed in parent-mediated interventions due to the cost, burden, and complexity of teaching parents to use multiple intervention strategies. To determine the optimal intervention sequence that considers parent moderators and parent use of intervention strategies, the investigators propose a sequential, multiple assignment, randomized trial (SMART) design in which the investigators will initially randomly assign 184 children with ASD, between 18 and 48 months of age, to receive either the SC or DB intervention. Following each respective manualized, 12-week intervention (first-stage intervention; SC or DB), the interventionist will measure the parents' use of intervention strategies. At this point, all parents will be re-randomized before starting the second-stage intervention. Second-stage intervention decisions are designed to be responsive to parents' implementation of the first-stage intervention strategies. That is, parents who are implementing the first-stage intervention strategies with high fidelity (high implementers) will be re-randomized to receive the same intervention at a lower frequency (Reduce) or to receive the other intervention (Switch). Parents who are implementing the first-stage intervention strategies with low fidelity (low implementers) will be re-randomized to receive the same intervention with an additional parent instructional method, such as video feedback (Augment) or to receive the other intervention (Switch). After 24 weeks of intervention (12 weeks for first stage, 12 weeks for second stage), the investigators will assess child SC skills, child DB, and family life participation in everyday activities. The investigators will also measure parent-child joint engagement continually during intervention to examine the extent to which joint engagement mediates intervention outcomes. The proposed research is significant because if an intervention for one domain (SC or DB) has an impact on the other, an intervention sequence that systematically includes both interventions may have an even greater impact on both domains.