Cells can die by two distinct pathways: apoptosis or necrosis. Necrosis is associated with rapid metabolic collapse that leads to cell swelling, early loss of plasma membrane integrity, and ultimate cell rupture. Cytosolic contents leak from the necrotic cell causing injury and inflammation to surrounding tissue. In contrast, apoptosis is an energy-requiring, gene-directed process, which, when activated, results in cell "suicide." The morphological and biochemical characteristics of cells dying by apoptosis differ markedly from those of cells dying by necrosis. During apoptosis, cells decrease in size and round up. The nuclear chromatin undergoes condensation and fragmentation. The apoptotic cell then breaks apart into many plasma membrane-bound vesicles called "apoptotic bodies," which contain fragments of condensed chromatin and morphologically intact organelles such as mitochondria. Apoptotic cells and bodies are rapidly phagocytosed, thereby protecting surrounding tissues from injury. The rapid and efficient clearance of apoptotic cells makes apoptosis extremely difficult to detect in tissue sections. Recent studies show that multiple cytotoxic stimuli well known to cause necrosis can lead to apoptosis instead when cells are exposed to the same noxious agents at lower concentrations. This insight has led to an interest in the role of apoptosis in the pathogenesis of renal diseases that result primarily from injury to renal tubular epithelial cells. These diseases include acute and chronic renal failure from exposure of the kidney to ischemia or to cytotoxic agents. In this review we discuss some relevant aspects of the differences between necrotic and apoptotic cell death. We also present evidence to support the hypothesis that apoptosis is an important pathogenic mechanism in those forms of acute and chronic renal failure in which the renal tubular epithelial cell is the primary target of ischemic or toxic injury.