Abstract All cells of the hematopoietic system have finite life spans, shorter by far than that of the host. They end their lives by committing a form of cellular suicide or programmed cell death. The morphology of this process is considerably different from that of necrosis and is called apoptosis. Apoptotic cells undergo a stereotyped sequence of changes, including shrinkage and nuclear collapse. The cell is quickly recognized and eaten by a phagocyte, without the elicitation of an inflammatory response. Although most cells have specific triggers of apoptosis, the killer T cell seems able to induce apoptosis in any cell it recognizes. The process of apoptosis is regulated by cytokines, and may be modulated both in vitro and in vivo. J. Leukoc. Biol. 57: 2–10; 1995.