gies, but variability in the nature and rate of development continues to be a source of confounding in the identification of disability (Simeonsson and Rosenthal, 2001). The lack of theory and the lack of consistent concepts of disability in childhood have resulted in identification and classification approaches that have been idiosyncratic to disciplines or service systems and taken the form of diagnoses, syndromes, or categories.

Although the definition and classification of disability in children are issues of current significance, they are not new problems. Concerns about diagnoses, categorical assignment, and the associated labeling of children with disabilities prompted a request for a coherent classification system in the early 1970s by then Secretary of Health, Education and Welfare Elliot Richardson (Hobbs, 1975a). In addition to the problem of labeling and its impact on children, the task of the project was also to address the inappropriate use of psychometric instruments and classification practices in the determination of disability in children from minority groups. The project resulted in two publications, with the first, Issues in the Classification of Children (Hobbs, 1975a), summarizing problems and challenges. In the second book, The Futures of Children, Hobbs (1975b) recommended a functional basis for the classification of children’s disabilities. The functional approach recommended by Hobbs was not realized, and the categorical basis for determining the eligibility of children for funding and services continues to raise concerns in the context of fragmented and incomplete services (Hughes et al., 1996; Newacheck et al., 1998).

Although the terminology for the concerns defined in the 1970s may have changed, many of the problems raised at that time remain the same today, requiring revisiting of the identification and the classification issues three decades later. The purpose of this paper is to (1) review concepts of development and disability; (2) present models and representative data on childhood disability; (3) describe current issues; and (4) identify emerging issues to advance the definition, measurement, and classification of disability in childhood.

Although there is no universal standard for defining childhood disability, a number of concepts have framed disability related to atypical development. In reviewing the research on different outcomes for children who have experienced significant developmental risk factors, Sameroff and Chandler (1975) described main-effect, interactional, and transactional models to capture the evolving findings on the roles of biological and environmental factors in child development. In the main-effect model, a nurturist view