Mental retardation, a developmental disorder with many causes, both known and unknown, is described in this program using the criteria of the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision (DSM-IV-TR). The information presented includes a discussion of the known causes, including intrauterine exposure to infections and toxins, trauma, and genetic triggers, such as chromosomal defects and inborn errors of metabolism. Included also is a brief overview of normal growth and development from birth to age five years. In addition, descriptions of co-morbidities associated with retardation are explored.