PROJECT ABSTRACT In response to RFA-MH-18-700, the goal of this collaborative R01 is to demonstrate the therapeutic value and community-wide implementability of an early intervention (EI) platform for toddlers with autism spectrum disorder (ASD) that is completely virtual, from recruitment through intervention. This platform—Early Social Interaction Mobile Coaching (ESI-MC) deploys individual telehealth sessions with coaching and feedback to help families embed intervention in everyday activities. Specifically, we will conduct an effectiveness trial of ESI-MC to address the important question of whether starting evidence-based intervention earlier leads to better outcomes than starting later. We will address this question by using a modified stepped wedge design and blended implementation research to analyze data obtained with ESI-MC start at 18, 24, or 30 months. We will diagnostically ascertain 240 children from a pool of 360 18-month-olds with early signs of autism, 60 in each of four US regions (Northeast, Southeast, Midwest, West Coast). They will be recruited using a new virtual platform—My Baby Navigator—linking a new surveillance and screening tool, an app to upload video-recorded home observations and telehealth intervention sessions, and a package of educational resources. The 240 children will be randomly assigned to one of three ESI-MC timing groups. We will measure child active engagement and social communication change every 6 months as the primary outcome variables. Outcome measures of developmental level, autism symptoms, and adaptive behavior will be examined to measure differential treatment effects. We will achieve these objectives through research AIMS: 1. Compare the effectiveness of ESI-MC implemented for 6 months on proximal outcome measures of child active engagement, child social communication change, parent transactional supports, and parent evidence-based strategy use (1A) with Treatment-as-Usual (TAU) at 24 and 30 months and (1B) across treatment timing groups initiated at 18, 24, or 30 months. 2. Examine (2A) change in parent transactional supports and evidence-based strategy use as the mechanism for change and (2B) individual child and family characteristics that moderate response to treatment. 3. Compare the effectiveness of intervention on secondary outcome measures of child developmental level, autism symptoms, and adaptive behavior (3A) with Treatment-as-Usual (TAU) at 24 and 30 months and (3B) across treatment timing groups initiated at 18, 24, or 30 months. 4. Explore outcomes at 36 months, individual patterns of change from 18-36 months, and predictors of change across treatment timing groups by estimating child growth trajectories. 5. Examine barriers and promotive factors impacting widespread dissemination, implementation and sustainability across racial, socioeconomic and geographic lines. Maximizing the use of mobile technology, ESI-MC offers the prospect of a community-viable, scalable and sustainable tre...