Cell death is a common phenomenon in developmental biology, and recent data suggest that it is as tightly regulated as mitosis. For numerous systems endocrine and neuronal factors are required to maintain viability of cells, as are specific diffusible and other unknown factors deriving from intimate cell-to-cell contact; and, in some instances, specific hormones or other circulating factors induce spontaneous self-destruction by the targeted cells. Some cells such as thymocytes may be primed to self-destruct and hence activate specific enzymes. In others, the doomed cell up-regulates a limited number of genes just before it dies. Of these genes, several are known but are not considered to cause cell death; others are under investigation. Although the situation is clearest for developmental biology, it appears that the presumptively random loss of cells in senescence results from invocation of the same mechanisms. Understanding and control of these mechanisms could conceivably lead either to protection against cell loss or specific induction of lysis in malignant cells.