The nephrology community has begun to recognize the importance of physical functioning in the overall treatment of their patients. Physical functioning is highly associated with such outcomes as hospitalization, nursing home admission, falling, level of dependency, and death in older individuals. Because there are many terms used to refer to physical functioning, this report classifies physical functioning into basic actions and complex activities; activities considered essential for maintaining independence, and those considered discretionary that are not required for independent living, but may have an impact on quality of life. We also present a model of the determinants of physical functioning, which goes beyond the presence or absence of disease and considers physical, sensory, environmental, and behavioral factors. Measurement of physical functioning can be complicated and ranges from self-report questionnaires to performance measures of specific tasks to vigorous laboratory measures. There are limitations to each of the measurement methods; however, some level of assessment provides information about the patient that is not otherwise available. Valid and reliable tests of physical performance are available that are easily administered and provide valuable information about the patient. Just as the patient's nutrition, medications, and adequacy of dialysis are monitored, baseline and subsequent physical functioning assessments allow us to monitor the patient's clinical course as it relates to their physical ability. Such measurement also allows for the identification of patients with lower functioning who would benefit from physical therapy or other exercise intervention.