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 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 30 miles from Stanford University;

   - Child's primary language other than English.