Cancer is a disease associated with aging. As the population ages worldwide, the number of older adults with cancer is dramatically increasing. In both the geriatric and oncology literature, an individual's functional status is one of the strongest predictors of overall survival and resource requirement. However, the measures traditionally used in oncology practice to assess functional status in patients of all ages--and to determine the course of treatment--do not identify the subtle degrees of functional impairment that predict morbidity and mortality in the geriatric population. This article describes the tools used by geriatricians to assess physical function, and outlines their prognostic significance for the patient with cancer. Including these tools in daily oncology practice could help physicians to better assess and treat vulnerable older adults.