This grant provides support for an NIH Autism Center of Excellence (ACE). This ACE aims to discover the mechanisms related to early brain overgrowth in autism and to define specific behavioral and biological factors associated with infants at-risk for the disorder. The Clinical Phenotype: Recruitment and Assessment Core (CPRAC) will implement a strategy called the 1-Year Well-Baby Check-Up Approach to identify and study infants at risk for an autism spectrum disorder (ASD) prospectively. This unbiased approach will identify ASD cases from both singleton and multiplex families, and the CPRAC will recruit a total of 3 study groups: at-risk ASD, at-risk developmental disability, and typically developing. The CPRAC will be responsible for: 1) identification and recruitment of 12-month old infants, 2) medical and behavioral diagnostic and psychometric evaluations of study infants every 6 months, 3) coordination of each infant's participation in magnetic resonance imaging (MRI), functional MRI, and genetic studies, and 4) final diagnoses when each participant turns three.