This study is part of an Autism Centers of Excellence (ACE) Network. Multiple lines of converging evidence (from magnetic resonance imaging (MRI), post-mortem, and head circumference studies) document brain enlargement in autism, with some evidence of this within the first year of life. New imaging methods provide highly detailed MRI data and are well suited for rapid scanning of very young children. In addition, new behavioral assessment tools enable the efficient screening and detection, at 12 months, of infants who are likely to meet criteria for autism at age two. In this study, these new methods will be used to conduct a longitudinal MRI/diffusion tensor imaging (DTI) study of six month old infant sibs of autistic individuals, with follow-up at 12 and 24 months of age. This research will provide new information about the trajectory of early postnatal brain overgrowth (regions, tissues, structures, and fiber tracts), as measured on MRI and DTI, and its potential relationship to clinical features. It may provide insights into developmental brain and behavioral phenotypes, as well as neurobiological mechanisms, that will inform other levels of analysis (e.g., molecular biological studies of this period) during what appears to be a critical period of development in the pathogenesis of autism.