Although it is generally acknowledged that comorbidity has a significant impact on treatment selection and outcomes for elderly patients with cancer, an understanding of how comorbid conditions should influence clinical decisions is quite incomplete. This issue remains an important and challenging area of geriatric oncology research. Measures of comorbidity require recognition, documentation, and accurate data extraction. Extensive prospective evaluations and medical chart reviews have the greatest reliability but are costly. Administrative data are convenient and applicable to large populations, but suffer from relatively poor reliability and therefore should be used with caution. Perhaps in the future more refined nosology and better information systems will improve our understanding and ability to use administrative data sets to study comorbidity in the elderly—the eventual goal of such investigation being evidence-based criteria for care of elderly cancer patients.