The National Institute on Aging (NIA) Geriatrics and Clinical Gerontology (GCG) Program convened an interdisciplinary Task Force on Comorbidity to foster the development of a research agenda on the multiple concurrent health problems that often occur in older persons. This report summarizes Task Force discussions held in Bethesda, Maryland (October 21–22, 2003; July 20–21, 2004) and serves as an introduction to the following three articles that address specific issues such as the nosological classification of impairment for the construction of comorbidity measures, staging and classification of disease severity, and methodological and analytical issues. The risk of developing concomitant chronic illnesses and physiological limitations escalates with aging. Diabetes, respiratory diseases, cancer, cardiovascular problems, arthritis, hypertension, and certain other chronic conditions are more common in older than in younger persons. As a consequence, a new diagnosis of any common chronic health condition is likely to be made in the context of preexisting health problems.