PROJECT SUMMARY/ABSTRACT Language outcomes are highly heterogenous in autism and can impact long-term psychosocial, educational, and vocational outcomes for children on the autism spectrum. Thus, there is a pressing need to identify novel approaches to language intervention, ideally those that can be implemented in early stages of development, when brain and behavior are most plastic. Many have begun to consider “pre-emptive” interventions for infants with autistic older siblings (Baby Sibs), who are highly likely be diagnosed with autism or developmental language disorder themselves. The Looking and Language Project (LoLa) seeks to (a) validate a theoretical model of language development in Baby Sibs, whereby looking to the mouth of a speaker increases later language skill via caregiver-child engagement and prelinguistic vocal complexity and to (b) systematically test the effects of a novel intervention that encourages looking to audiovisual speech by moving the referent of interventionist talk near the mouth (Milieu Teaching-AV). I will utilize the K99 portion to conduct a longitudinal correlational study with 52 Baby Sibs and follow them from 6 months to 15 months. I will additionally use this period to evaluate the feasibility of Milieu Teaching-AV with this population and assess parent attitudes towards and perceived barriers of “pre-emptive” interventions for Baby Sibs. The training portion of the K99 will focus on (a) mastering strategies necessary to train coaches to fidelity on intervention procedures, (b) developing expertise in implementation science methods, and (c) mastering advanced methodological and statistical approaches to randomized controlled trials (RCTs). During the R00 phase, I will evaluate the effects of Milieu Teaching-AV compared to a business as usual control in 60 Baby Sibs within the context of an RCT. This RCT will examine direct effects on (a) looking to the mouth of a speaker, (b) caregiver-child engagement, (c) prelinguistic vocal complexity, and (d) expressive and receptive language. The RCT will also allow for an exploratory look at moderated effects (i.e., determining for whom the intervention yields optimal outcomes) of the intervention on the basis of (a) language ability and (b) caregiver-child engagement at study entry and (c) preliminary autism diagnosis at 15 months. Additionally, I will evaluate the acceptability of the intervention and outcome measures for key interested parties, including adolescent and adult siblings of autistic individuals (i.e., grown-up Baby Sibs), autistic-self advocates, caregivers of autistic children, and community early intervention providers. NOVELTY AND IMPACT: If my hypotheses are born out, the proposed project will provide empirical support for a novel approach to preemptive intervention that is acceptable to key interested parties and motivated by theory and a large extant literature involving autistic children. The results of the RCT will provide the groundwork for ...