This presentation will focus on studies validating social visual engagement, the moment-by-moment way children look at and learn about their social surroundings, as a quantitative biomarker for autism. Leveraging this science, we have now developed and validated an eye-tracking-based tool for the diagnosis and assessment of autism in 16-30-month-old toddlers. In two multisite, prospective, double-blind clinical trials involving >1,600 toddlers, including three independent cohorts and three replications, this tool showed accuracy of a quantitative diagnostic classifier and of three quantitative indices of severity: social disability proxying the total score of the ADOS-2, and verbal and nonverbal age equivalents proxying the verbal and nonverbal scales of the Mullen. This tool was cleared by the FDA in July 2023 and has been in clinical use in the United States since August 2023. Results of the trials appeared in simultaneous publications in JAMA and JAMA Network Open in September 2023.