As their patients age, clinicians are increasingly confronted with diseases, impairments, or conditions that complicate the diagnosis and treatment of index diseases and with the management challenge of the overall burden of morbidity in their patients. Nevertheless, there has been little applicable conceptual work and empirical research on the nature, impact, and management of comorbid and multiple morbid conditions. Advancing research in this area has been the goal of a recent National Institute on Aging Task Force on Comorbidity Research Issues, as well as a working conference of the American Geriatrics Society. Both efforts highlighted current knowledge and thinking about comorbidity in the context of caring for an aging population, in preparation for research ultimately leading to better understanding of comorbidity and improvement in clinical practice. This Perspective provides a brief overview of these developments.