Renal insufficiency is defined for this discussion as a defect in excretory function that interferes with well-being or even places life in jeopardy. The acute insufficiencies result from more or less selective toxic or ischemic damage to tubule cells, they are usually associated with oliguria or anuria, and their course is toward recovery by cell regeneration; hence, the aim of treatment is to maintain the patient until this predictable repair has been adequately effected. In contrast, in chronic renal insufficiency, blocks and masses of nephrons have usually been destroyed beyond repair. Many patients with chronic renal insufficiency can be maintained in comfortable and useful life in spite of severe defects in renal function, but treatment is essentially palliative and the ultimate prognosis commonly grave. The causes, lesions, courses, and therapeutic aims are different in acute and chronic renal insufficiency. In both, however, the basic principles of treatment are the same.