Previous studies which assess brain function in adults with autism using a technique called functional magnetic resonance imaging (fMRI) have defined the neural circuitry involved in a range of cognitive and psychological processes. These investigations have demonstrated the potential for fMRI studies to contribute to our understanding and treatment of autism. However, the volume of research to date using fMRI procedures to study children with autism has been minimal. This is unfortunate because fMRI is well suited to studying autism's developmental nature. Researchers have theorized that a common characteristic of individuals with autism is the striking abnormalities in social perception, that is, the evaluation of intentions of others by eye-gaze direction, facial expressions, body movements, speech or other kinds of motion. This is part of a domain of skills referred to as theory of mind, social cognition, mentalizing, or mindreading. This is a proposal to conduct a highly innovative combination of fMRI and eye tracking studies aimed at a more comprehensive characterization of the neural circuitry underlying the development of different aspects of social perception. Significance: This work will likely impact the development of new methods of diagnosis, treatment, and/or prevention of autism spectrum disorders by identifying neurophysiological mechanisms involved in social perception deficits. This knowledge will help inform skill-based treatment models that train teaching children with autism to monitor eye gaze, identify emotions, and infer the intentions of others. Finally, by defining functional brain phenotypes using fMRI, these studies have the potential to help differentiate subtypes of the autism spectrum, and provide clues into other causes of this disorder.