Recent randomized studies have failed to demonstrate that parent-implemented play and communication-based interventions delivered in typical home routines significantly accelerate developmental progress in young children with autism. However, there is solid evidence that the same types of procedures implemented by paraprofessionals in homes do accelerate child progress significantly compared to controls. The hypothesis is that parents are not being provided with sufficient training and support to embed enough high quality learning opportunities into daily routines to accelerate their children's progress. The goals for the project are: (Aim 1) to add novel features from the science of adult behavior change to enhance parent delivery of interventions at home; (Aim 2) to examine the causal relationships between enhanced quality and quantity of parent-implemented intervention on children's core and associated symptoms of ASD; and (Aim 3) to demonstrate the acceptability and effectiveness of these novel features via direct measurement of parental use at home. The proposed supplemental funding supports an enhanced study design across the two sites, including increased clinician time for coaching and interaction with families in both treatment groups, additional test instruments, and more rigorous evaluation during the time period that the families are involved in the study.