This two-part issue of Pediatric Clinics of North America examines Developmental Disabilities. Part I of this issue reviews topics including Global Developmental Delay and Mental Retardation:: Conceptualization, Evaluation and Causation; Dysmorphology Examination; Behavioral Phenotypes of Common Genetic Syndromes; Down Syndrome; Clinical Approach to the Diagnosis of Inborn Errors of Metabolism; Autism:: Definition, Neurobiology, Screening, and Diagnosis; Autism Spectrum Disorders in Adolescents:: Psychosocial Aspects and Management; Learning Disabilities:: Definitions, Epidemiology, Diagnoses, and Intervention Strategies; Concepts of Dyslexia in Children; Speech Disorders; and Use of Psychotropic Medications in Children with Developmental Disabilities.