The regulation of cell proliferation and the selection against autoreactive cells in the lymphoid system both occur through the induction of apoptosis. Many of the signals that induce apoptosis in lymphocytes are now well defined. Interactions between Fas and its ligand have emerged as a major mechanism for the deletion of activated peripheral T cells and autoreactive B cells. Although the signal-transduction pathway leading from engagement of Fas to apoptosis is not entirely clear, significant advances have been made recently. There has also been progress in the elucidation of the mechanisms that regulate apoptosis in the immune system.