PROJECT SUMMARY The rising number of children being identified as autistic has led to exponential growth in for-profit applied behavior analysis (ABA) agencies serving them, the use of highly structured approaches that may not be developmentally appropriate for young children1 and that limit use of learned skills across settings and time, and growing public health concerns regarding limited effectiveness data combined with the cost of services. Efficacy testing has led to growth in evidence-based autism interventions (EBI), especially naturalistic developmental behavior interventions (NDBIs), supported by multiple clinical trials. NDBIs integrate theory and strategies from ABA and developmental science and are considered best practice for young autistic children. The lack of effectiveness data regarding NDBI use in community-based agencies (CBAs) contributes to limited funding as payors are more likely to recognize older, structured methods. The Early Start Denver Model (ESDM) is a comprehensive NDBI shown to improve social communication and language outcomes for young autistic children in multiple controlled efficacy studies. ESDM engages social motivation as a mechanism to increase child engagement in social learning opportunities in the environment resulting in increased rate of child learning. ESDM includes assessment and data collection methods that meet funder requirements and a tested community training model. Understanding whether ESDM is effective and whether the same treatment mechanisms operating in efficacy trials are also operating in community implementation with diverse samples are critical scientific questions to determine the potential of NDBIs like ESDM to meet the public health goals of improving access to high quality, developmentally appropriate care for a broad range of young autistic children and their families. To facilitate scale up, we have partnered with autism CBAs and this study will identify factors that affect implementation of ESDM in the community. This project will use a hybrid type 1 randomized controlled design to examine ESDM effectiveness in the community and to gather data on implementation determinants. Primary child outcomes include language and social communication. Secondary outcomes include adaptive behavior, parent use of ESDM strategies, and provider fidelity to the model. Social motivation and caregiver fidelity will be measured as mediating variables. The moderating effects of maternal education, child race/ethnicity, and provider ESDM fidelity will be examined. ESDM implementation determinants will be explored. Understanding the effectiveness of an intervention like ESDM, the variables that mediate and moderate child outcomes, and engagement of its mechanisms of action in community use, has the potential to increase availability, and therefore, access to high quality intervention for all young autistic children, especially those from diverse backgrounds who depend on public services. Understanding ...