The aging of the population and the emergence of cancer as a major disease of older age require the development of new strategies of cancer management in older adults, in particular, strategies based on the special characteristics of older patients, such as the effects of concurrent health conditions, i.e. comorbidity. This paper is a consideration of the implications of current research on comorbidity for research and practice in geriatric oncology. Subjects include considerations of representative measures of comorbidity; sources of data; screening and early diagnosis; prognosis, treatment, and outcome; and physiological mechanisms. Recommendations for future research include the development of (a) more refined measures of comorbidity; (b) new studies of comorbidity and cancer treatment; (c) investigations of the contributions of comorbidity for geriatric assessment and a new generation of clinical trials, (d) examinations of physiological mechanisms linking comorbidity with health outcomes; and (e) the development of an epidemiology of comorbidity.