Since the early 1990s, there have literally been thousands of reports related to magnetic resonance imaging of the autistic brain. The goals of these studies have ranged from identifying the earliest biological predictors of an autistic diagnosis to determining brain systems most altered in autistic individuals. Some of the later works attempt to use distinct patterns of brain alterations to help define more homogenous subtypes of autism. Far less work has been done to identify brain changes that are associated with therapeutic interventions. In this chapter, we will touch on all of these efforts as they relate to the general topic of the usefulness of brain imaging as a biomarker of autism.