To better understand the health of older people, it is valuable to go beyond conventional measures of disease and self-perceived health and utilize assessments of functional status and disability. Physical decrements can be characterized at the level of organs and body systems (impairments); the organism as a whole (functional limitations); and the person in the context of the environment and its challenges (disability). To understand the dynamics of the pathway leading from disease to disability, it is critical to measure functional limitations that can be assessed through either self-report or standardized objective measures of physical performance. Both of these approaches may be used to evaluate functions such as grasping objects, walking, and climbing stairs. In aging research, measures of functional limitation are utilized as outcomes that indicate the impact of disease, impairments, and other risk factors on function. In turn, measures of functional limitation can be used to characterize the functional status of individuals and populations, and are powerful predictors of various adverse outcomes, including incident disability in people not currently disabled. Functional limitation measures add substantially to our knowledge about older populations, but further work is needed to promote their standardization and use in both clinical and research settings.