A host of abnormalities of renal structure and function accompanies advancing age. An appreciation of methodologic considerations, including population selection, that might confound the assessment of the effects of aging on renal function has prompted a recent reappraisal. Earlier studies assessed the effects of aging by utilizing cross-sectional studies and institutionalized elderly subjects, with their attendant drawbacks. Recent longitudinal studies have utilized appropriate patient cohorts, selected for lock of renal disease, including potential kidney transplant donors. These studies indicate that the morphological and functional changes of aging tend to be less marked than previously thought. The common denominator of these functional changes is a diminution in renal reserve, along with constraints on the kidney's ability to respond appropriately to challenges of either excesses or deficits. Although these alterations are unlikely to be of major clinical consequence under everyday conditions, they attain clinical significance when residual renal function is challenged by the superimposition of an acute illness. Finally, it should be emphasized that elderly patients frequently suffer from comorbid conditions, such as hypertension and heart disease, that may be additive to the changes of aging, thereby amplifying these abnormalities.