The field of rehabilitation is becoming increasingly important as the American population ages, seriously ill and injured individuals survive with impairments, and quality of life assumes greater importance in health outcomes assessment. Like other branches of health care, progress in rehabilitation depends on advances in research. However, the rehabilitation process is different from some other facets of health care in that it focuses simultaneously on health outcomes that range from cellular to social. This fundamental difference affects the research methodologies that are appropriate to the field. The strengths and limitations of existing research methods need to be taken into consideration and novel strategies developed to handle the complexity inherent in rehabilitation practice.