This review of adaptive changes in renal structure and function in subjects with reduced renal mass has two primary goals. One is to provide a description of the remarkable compensatory increases in glomerular filtration rate (GFR), and renal blood flow, at the level of individual nephrons, and the alterations in water and electrolyte transport by tubular epithelium. These processes preserve fluid and electrolyte balance in subjects with progressive renal failure, until whole kidney GFR is reduced to about 20 percent of normal, and provide the basis for conservative clinical medical management. The other aim is an attempt to provide an understanding of the mechanisms involved in compensatory adaptation, since this information, in addition to amplifying our understanding of renal transport processes, helps to elucidate the functional limitations placed on subjects with renal insufficiency. An attempt has been made to analyze both clinical observations and relevant experimental models and an effort has been made to correlate renal function with different patterns of renal injury.