Americans' perceptions of childhood disability have changed dramatically over the past century, as have their ideas about health and illness, medical developments, threats to children's health and development, and expectations for child functioning. Neal Halfon, Amy Houtrow, Kandyce Larson, and Paul Newacheck examine how these changes have influenced the risk of poor health and disability and how recent policies to address the needs of children with disabilities have evolved. The authors examine the prevalence in the United States of childhood disability and of the conditions responsible for impairment, as well as trends in the prevalence of chronic conditions associated with disability. They find that childhood disability is increasing and that emotional, behavioral, and neurological disabilities are now more prevalent than physical impairments. They stress the importance of, and lack of progress in, improving socioeconomic disparities in disability prevalence, as well as the need for better measures and greater harmonization of data and data sources across different child-serving agencies and levels of government. They call on policy makers to strengthen existing data systems to advance understanding of the causes of childhood disabilities and guide the formulation of more strategic, responsive, and effective policies, programs, and interventions. The authors offer a new and forward-looking definition of childhood disability that reflects emerging and developmentally responsive notions of childhood health and disability. They highlight the relationship between health, functioning, and the environment; the gap in function between a child's abilities and the norm; and how that gap limits the child's ability to engage successfully with his or her world. Their definition also recognizes the dynamic nature of disability and how the experience of disability can be modified by the child's environment.