Autism spectrum disorder (ASD) is a serious neurodevelopmental disorder defined by impaired social engagement and social communication, in addition to the presence of repetitive, restricted, or stereotyped behaviors and interests. Although many cases of ASD can be detected when children are less than two years old, the average age of diagnosis in the US is still after the fourth birthday. However, evidence demonstrates that children who start ASD-specific early intervention have better outcomes than children who do not start treatment until later ages. In 2006 and 2007, American Academy of Pediatrics recommended three early detection approaches to improve identification of children at risk for ASD: ongoing developmental surveillance at every well-child check-up, routine broad developmental screening at three infant/toddler ages, and ASD-specific screening at two toddler ages. When these early detection strategies are used with all children attending well-child check-ups, the age of ASD detection is lower, and children who are diagnosed have the opportunity to start ASD-specific early intervention at younger ages than if they had not been detected. Yet in 2016, the US Preventive Services Task Force (USPSTF) indicated that current evidence is insufficient to recommend universal ASD screening, given the lack of experimental studies demonstrating positive outcomes for treated children that are detected through screening. The current study will address this gap. This study will directly relate early detection strategies to early intervention, and measure the impact of age of intervention onset on outcomes when children are entering kindergarten. The study will be conducted by investigators from three sites: Drexel University; the University of California, Davis; and the University of Connecticut. Local pediatric providers will be enrolled in the study, and their practices will be randomized to provide either usual care, or to an experimental condition in which autism early detection strategies are enhanced through the addition of specific procedures. Children attending well-child visits at participating practices will then be enrolled. Across all sites, 8,000 children will be recruited through their participating pediatric practice. As part of the study, qualifying children will receive up to one year of early intensive behavioral intervention, using an evidence-based manualized treatment. Primary outcome measures will include children's cognitive functioning and ASD symptom severity, which will be measured at multiple time points. Exploratory outcomes will include children's adaptive functioning, kindergarten readiness, and social reciprocity, as measured by experimental eye tracking and parent-child interaction ratings. This study also will examine the impact of the screening intervention on physician attitudes and on parent empowerment and stress. Finally, investigators will examine potential moderators of outcomes, to determine whether initial symptom severity, cognitive ability, or socioeconomic status affects children's long-term outcomes. The investigators predict that this study will inform early detection strategies which will result in improving children's social and cognitive functioning, mitigating lifespan disability, reducing societal costs, and improving personal well-being and productivity of individuals with ASD.