Parent Training for Latinx Autistic Toddlers: Development and Preliminary Testing - PROJECT SUMMARY Latinx children with diagnosed or suspected autism have especially long wait times for diagnoses and intervention, up to a year or even more. This delayed access to intervention is likely to prevent children from reaching their full potential in skills and adaptive functioning. A comprehensive, online mobile device-based, free intervention that can be used by parents while on these wait lists may optimize their child's development. We have developed such a program, the Online Parent Training in Early Intervention (OPT-In-Early) program. It teaches parents the fundamental principles of both applied behavior analysis and naturalistic interventions, uses non-technical language, video demonstrations, guidance in selecting appropriate skills to teach, embedding teaching into daily routines, and reducing interfering behaviors. It is a self-paced, individualized platform giving parents strategies to strengthen the parent-child relationship, to teach basic skills, such as simple communication, to reduce interfering behaviors, and to establish helpful routines. In pilot the RCT, parents (half of whom were self-identified as Latinx) rated the program very highly and showed positive changes in behavior and knowledge. The goal of the current project is to develop, and pilot test a bilingual Spanish/English culturally tailored version of OPT-In-Early for Latinx families in the US. First, we will obtain detailed, iterative feedback from bilingual Latinx parents with an autistic child regarding OPT-In-Early. We will use this feedback to culturally adapt the program content and format. Next, we will modify the adapted program via user testing with Spanish- speaking parents. Finally, we will test the bilingual version in a six-month pilot RCT, in preparation for a fully- powered R01 trial. We will use a wait-list control design: the Intervention Now arm will be given the online program and the Intervention Later arm (controls) will be given educational material about autism. All children will receive a baseline and 6-month follow-up assessment, which will measure parent fidelity in using the intervention principles (the primary outcome), child social communication, level of autism symptoms, impact of autism symptoms, and degree of developmental delays. At the conclusion of this research, we will have developed an evidence-based, comprehensive, freely available, parent-delivered intervention that can be used by US Latinx families on their mobile devices as they await diagnosis and intervention. This will increase parent efficacy and reduce their stress, accelerate children's skills, reduce interfering behavior, and enhance PCPs' willingness to implement universal autism screening.