Autism spectrum disorder (ASD) is a complex disorder for which current treatments are limited. It is defined by social deficits and the presence of restricted and repetitive behaviors, but the majority of patients also show one or more comorbid conditions. This diversity suggests that multiple distinct mechanisms likely underpin the many clinical issues of ASD. One approach to determining these mechanisms is to identify discrete, fundamental components of the complex clinical phenomena that define ASD, and then clarify the neural bases of these components. Motor behaviors are particularly promising for this purpose because: 1) They are critical building blocks for social and cognitive development; 2) motor deficits are common in ASD and associated with worse social, cognitive, and daily living skills, and; 3) they are supported by neocortical, striatal, and cerebellar brain networks implicated in post-mortem and magnetic resonance imaging (MRI) studies of ASD. The proposed studies will test the hypothesis that fundamental motor deficits and their neural processes are related to discrete clinical symptoms of ASD. Individuals with ASD are predicted to show deficits during continuous motor behaviors in which they must reactively adjust motor output in response to sensory feedback. They also are expected to show increased rates of initial force production and motor dysmetria during rapid motor behaviors. Continuous motor deficits are hypothesized to be selectively associated with abnormal activation in and functional connectivity of cerebellar-cortical processes. Rapid motor deficits are expected to be selectively associated with abnormal activation in and functional connectivity of striatal-cortical processes. The proposed studies will identify the clinical issues associated with these motor-brain deficits in order to develop a more mechanistic understanding of ASD that can guide the development and validation of treatments targeting fundamental component processes rather than complex clinical symptoms.