Apoptosis is a physiological process of cell death that occurs as part of normal development and in response to a variety of physiological and pathophysiological stimuli. The effector mechanisms which carry out the death program are well preserved across species and evolution. Apoptosis is important in the immune system, and plays significant roles in the control of the immune response, the deletion of immune cells recognising self-antigens, and cytotoxic killing. Some of the molecular regulators of these processes, such as CD95 and bcl-2 family proteins are the subjects of intense research. Malfunctioning of the immune system may lead to increased or decreased cell death. Conversely, dysregulation of apoptotic pathways themselves may lead to a spectrum of human disease, including autoimmune disease and immunodeficiency.