Autism is a neurodevelopmental disorder characterized by pervasive deficits in language, behavior, and cognition. Pathology exists throughout the brains of subjects with autism including the cerebellum. These changes include changes in cerebellar and vermal volume, changes in pyramidal cell density, and changes in gray and white matter. Additionally, a number of brain markers associated with GABAergic function, brain development, inflammation, immune system function, and apoptosis have shown altered expression in the cerebellum of subjects with autism. The initial focus was on the contribution of cerebellar pathology on movement disorder. Over the past 15 years there has been an increased understanding that the cerebellum is involved in emotional processing, cognition, and other higher brain functions, many of which are impaired in autism. Ataxia, or abnormal gait, is often accompanied by degeneration of the cerebellum. Moreover, similar to autism, Ataxia is often associated with deficits in executive function, emotional processing, and cognition. The purpose of this chapter is to summarize findings of cerebellar pathology in autism and how cerebellar pathology may contribute to the behavioral and cognitive aspects of autism and Ataxia.