The intercellular connections that characterise advanced forms of life would not be possible without a mechanism to remove individual cells that are no longer needed, or that function abnormally. Such physiological cell death, in the absence of inflammation, is achieved by apoptosis, a structurally distinct programmed cell death pathway. Defective regulation of programmed cell death may play a part in the aetiology of cancer, AIDS, autoimmune diseases, and degenerative diseases of the central nervous system. The pharmacological manipulation of apoptosis offers new possibilities for the prevention and treatment of these illnesses.