Cell death can occur by two possible mechanisms, necrosis or apoptosis. Necrosis is the classically recognised form of cell death and is characterised by high amplitude swelling of the mitochondria, nuclear flocculation and uncontrolled cell lysis. Tissue necrosis is normally seen following severe trauma to cells. The alternative form of cell death is via a programmed sequence of events and is termed apoptosis. Apoptosis occurs under a variety of physiological conditions and cells dying by this process undergo cytoplasmic and nuclear condensation, coupled with cleavage of the cell's DNA into nucleosome size fragments. DNA cleavage is due to the activation of a specific endogenous endonuclease. The cell finally fragments into apoptotic bodies which are engulfed by neighbouring cells and degraded. Apoptosis is an energy requiring process requiring intact energy generating systems, unlike that of necrosis. In relation to malignant disease, apoptosis is the mechanism by which cytotoxic T cells kill tumour target cells; it may also be the mechanism which accounts for the high loss of cells in growing tumour masses. Extensive apoptosis is seen in regressing tumours and also in those treated with chemotherapeutic agents. This form of death may require the activation of specific death genes, although in view of work carried out in this and other laboratories, demonstrating that inhibitors of both protein and RNA synthesis will readily induce apoptosis, this may not be universal. Finally, apoptosis has far reaching implications for the treatment of malignant disease, since only by understanding how cells die will be able to develop more effective means of killing them.