Autism was first recognized as a pervasive developmental disorder (PDD) in the mid-twentieth century when Leo Kanner, M.D. noted 11 children engaging in atypical behaviors that were not consistent with other identified psychiatric conditions such as schizophrenia. Kanner observed three areas of impairment in these children including abnormal language, insistence on sameness, and social isolation, and is credited with labeling this constellation of symptoms as infantile autism. Wing and Gould (1979) noted similar symptoms approximately 30 years later and proposed the "triad of impairment," which conceptualized autism as a disorder characterized by impairment in reciprocal social interaction, verbal and non-verbal communication, and a restricted repertoire of activities or interests. The current diagnostic criteria for autism reflect these general categories and include qualitative impairment in social interaction (a minimum of 2 symptoms), communication (a minimum of 1 symptom), and a restricted repertoire and repetitive stereotyped pattern of behaviors, interests, or activities (a minimum of 1 symptom) with a total of 6 symptoms needed to meet full criteria [American Psychiatric Association (APA), 2000]. The current chapter focuses on symptoms characteristic of the latter category with an emphasis on the types of motor abnormalities manifested in children with autism, the function of these behaviors, and the underlying biological mechanisms to explain the occurrence of these behaviors. Symptoms encompassing this latter category, including a restricted and stereotyped range of activities and interests, have typically been conceptualized as behaviors that are observable and atypical. The APA lists a cluster of symptoms representative of this category in the Diagnostic and Statistical Manual of Mental Disorders, 4th edition text revision (DSM-IV-TR; APA, 2000). The first symptom listed is a preoccupation with one or more stereotyped and restricted patterns of interest that is abnormal either in intensity or focus. This may be manifested in seeking out and retaining vast amounts of information about one topic such as geography or presidents or collecting information about people's birthdates. It is important to note that up to 75% of children with autism also have comorbid mental retardation and that this symptom tends to be more common in high functioning children with autism who do not score in the range of mental retardation on measures of intellectual functioning. A second symptom of this category is the compulsive adherence to nonfunctional routines and rituals. Children with autism may have a need to follow a particular route to school each day or complete daily tasks in a particular order. If they are prevented from engaging in their routine they exhibit distress which can escalate to physical resistance. A third symptom characteristic of this category as listed in the DSM-IV-TR is a preoccupation with parts of objects at the exclusion of recognizing and utilizing objects in a functional manner. A child with autism may be satisfied spinning the wheels of a toy car repetitively instead of rolling it on the ground or racing it with another car. Or a child may be preoccupied with and focus on the spinning blades of a ceiling fan or repetitively pressing buttons on a remote control rather than using the objects in a functional manner. © 2005 Elsevier Inc. All rights reserved.