Developmental delay is frequently used to identify children with delay in meeting developmental milestones in one or more streams of development. There is no consensus on the specific definition. Developmental delay is best viewed generically as a chief complaint rather than a diagnosis. A child suspected to have delays should always be assessed in each of the major streams of development: expressive and receptive language, including social communication; visual problem solving (nonverbal cognition); motor development; neurobehavioral development; and social-emotional development. A model developed by the National Center for Medical Rehabilitation Research is used to compare existing classifications of developmental delays. This model defines the five domains in the disability process: pathophysiology, impairment, functional limitation, disability, and societal limitation. An etiology domain is added. This model is used to illustrate how existing classification systems of cerebral palsy, mental retardation, autism, and language delay draw on information from one or more domains. The model illustrates some of the conflicts between different systems. For example, most classification systems for cerebral palsy emphasize only impairment (spasticity, dyskinesias, and topography). The current definition and classification system for mental retardation focuses on functional limitations (IQ), disability, and societal limitations, ignoring pathophysiology and details of impairment. Given the complexity of neurodevelopmental disabilities, it is unlikely that a single classification system will fit all needs.