While significant progress has been made toward identifying effective interventions for preschool-age children with autism, few scientifically rigorous studies have compared the essential elements of these interventions or examined outcomes focused on core deficits. To address these areas of need, this collaborative, multi-site project combines the expertise of investigators experienced in randomized controlled clinical trials (RCTs), in the study of core deficits in young children with autism, and in data management and analysis of multi-site clinical trials. The goal of this project is to compare the efficacy of two interventions for improving spoken language and reducing symptoms of autism: (1) Discrete trial training (DTT), which is an applied behavior analysis approach emphasizing learning readiness skills and (2) Interpersonal developmental approach (IDA), which is a child-focused approach that focuses on social engagement. Each site will recruit 64 children with autism, 3-4 years of age, who are engaged in 25 hour per week early intervention programs. This intervention will be augmented with two 30-minute sessions daily of the study intervention (DTT or IDA) conducted by supervised therapists for 4 months, with transition to home therapy for 2 months. The primary aims are to compare intervention conditions (DTT and IDA) on child outcomes of spoken language and core deficits of social features of autism both at the end of treatment and 6 months later, as well as to examine potential moderators (e.g., mental age, language age) on treatment outcome. Secondary exploratory analyses focus on potential mediators (parent synchronization of joint attention and changes in parental expectancies) and non-specific factors (quality and quantity of community treatment).