This Element provides a broad overview of autism spectrum disorder from early childhood through adolescence. The Element reviews high-impact areas of research relevant to young children, including the shifting diagnostic conceptualizations of autism, current best practices related to screening and diagnosis, our understanding of factors that increase the likelihood of receiving an autism diagnosis, the overlap between autism and other co-occurring conditions, and related contemporary approaches to supports and interventions for young children. The discussion of these topics addresses measurement of outcomes, reproducibility, and methodological rigor. By focusing on these methodological gaps and progress, future directions for research in each of these areas is highlighted.