Stroke remains one of the major public health problems in the United States today, with approximately 500 000 new or recurrent cases occurring each year. About 4 000 000 persons alive today have survived a stroke and have some neurological deficits. Although the magnitude of healthcare resources used to treat and rehabilitate stroke survivors is considerable, to date a standardized, comprehensive classification system to document the resultant impairments and disability has not been developed. Successful management of any disabling disease, including stroke, should benefit from the use of a classification system to judge the impact of treatment, particularly emerging therapies. Participants in the Methodologic Issues in Stroke Outcome Symposium determined that the complex nature of stroke recovery demands clarification of its natural history and classification of the variable patterns of functional recovery. For stroke survivors to receive the best care, a comprehensive stroke outcome classification system is needed to direct appropriate therapeutic interventions. Building on the work and recommendations of the Stroke Outcome Symposium, the American Heart Association Classification of Stroke Outcome Task Force has worked to develop a valid and reliable global classification system that accurately summarizes the neurological impairments, disabilities, and handicaps that occur after stroke. The development of a stroke outcome classification system is predicated on the belief that neurological deficits often lead to permanent impairments, disabilities, and compromised quality of life. Although a person’s ability to complete daily functional tasks is thought to be largely dependent on and often limited by the type and degree of impairment, additional factors are often relevant in the ultimate determination of functional outcome. Thus, a classification of stroke outcome should include the broad range of disabilities and impairments as well as the relationship of disability and impairment to independent function. It is important to underscore that impairment alone does not define level of disability. In a study of stroke survivors it was determined that although a disability is most directly influenced by impairments, current stroke scales that measure impairments only partially explained the level of disability, handicap, or quality of life for those surviving at least 6 months. Some persons adapt well to many and/or severe impairments caused by stroke. Others with only minimal neurological impairments can be severely disabled. Many factors determine function, including the influence of poststroke rehabilitation training and the physical and social environments.