With global aging and scientific advances extending survival, the number of adults experiencing multiple chronic conditions has grown substantially and is projected to increase by another third between 2000 and 2030. Among the many challenges posed by multimorbidity, some of the most pressing include how to characterize and measure comorbid conditions, understand symptoms and illness burden, and provide person-centered care in the context of competing health care priorities and increasing complexity. In this white paper emanating from a National Institute on Aging supported conference to discuss research gaps at the geriatrics-palliative care interface, the authors review common definitions of multimorbidity; describe the association between multimorbidity and quality of life, functional status, quality of care, and health care utilization; note content and methodological gaps in multimorbidity evidence; and make recommendations regarding research priorities in this area of expanding public health impact.