Inclusion Criteria:  Diagnosis of Autism Spectrum Disorder (ASD) based on Autism Diagnostic Interview Revised (ADI-R), Autism Diagnostic Observation Schedule, Second Edition (ADOS-2) or Childhood Autism Rating Scale, Second Edition (CARS-2), Diagnostic and Statistical Manual 5th Edition (DSM-5), and expert clinical opinion; Boys and girls between 2.0 and 5.11 years; Ability to participate in the testing procedures to the extent that valid standard scores can be obtained; Language delay as measured by the Preschool Language Scale, 5th Edition (PLS-5): Standard score at least 1 standard deviation below average for expressive language ability for 2 and 3 year olds; 2 standard deviations for 4 year olds, and 3 standard deviations for 5 year olds; Stable treatment (e.g., Applied Behavior Analysis - ABA), speech therapy, school placement, psychotropic medication(s) or biomedical intervention(s) for at least 1 month prior to baseline measurements; No anticipated changes on treatment during study participation for Center-Based Pivotal Response Treatment (PRT-C) and In-Home Pivotal Response Treatment (PRT-H); No more than 60 minutes of individual 1:1 speech therapy per week; Availability of at least one parent who can consistently participate in parent training and research measures.  Exclusion Criteria:  Current or lifetime diagnosis of severe psychiatric disorder (e.g., bipolar disorder, etc.); Receiving in-home ABA of 15 hours or more Presence of active medical problem (e.g., unstable seizure disorder or heart disease); Previous adequate Pivotal Response Treatment (PRT) trial; Participants living more than 40 miles from Stanford University.