Apoptosis is a morphologically distinct form of programmed cell death that plays a major role during development, homeostasis, and in many diseases including cancer, acquired immunodeficiency syndrome, and neurodegenerative disorders. Apoptosis occurs through the activation of a cell-intrinsic suicide program. The basic machinery to carry out apoptosis appears to be present in essentially all mammalian cells at all times, but the activation of the suicide program is regulated by many different signals that originate from both the intracellular and the extracellular milieu. Genetic studies in the nematode Caenorhabditis elegans and in the fruit fly Drosophila melanogaster have led to the isolation of genes that are specifically required for the induction of programmed cell death. At least some components of the apoptotic program have been conserved among worms, insects, and vertebrates.