This article presents basic findings, concepts and methods in medical outcomes research. Epidemiologic outcomes research begins with the daily life of patients or populations and searches for antecedents. Clinical outcomes research begins with an intervention and examines effects. Both are needed. Issues and methodologies in medical rehabilitation outcomes research are examined, and topics for the forthcoming series are sketched. Medical rehabilitation can make substantial advances in understanding its effectiveness in the near future if it will undertake the needed comparative outcomes research