Physiological mechanisms of cell death are used by multicellular organisms for development and morphogenesis, to control cell number, and as a defensive strategy to remove infected, mutated, or damaged cells. Cell death is required for the normal development of almost all multicellular organisms and commonly involves the production of excess cells and removal of those that are superfluous. Frequently in plants, and occasionally in other organisms, cytoplasmic or structural components of the dead cell serve important functions. The process of cell death used by metazoans for development is highly conserved and is morphologically recognizable as apoptosis. The evolution of multicellularity and cell specialization brought with it a need for the regulation of cell death by intercellular signaling. The mechanisms that implement developmental apoptosis may have originated from cell-autonomous cell death processes used for defense. While the apoptosis effector mechanisms have been extensively characterized, understanding of the pathways that signal and control developmental cell death is far from complete.

When a cell in an organism dies due to a process encoded by that organism for the purpose of killing its own cells, that death can be considered to be a physiological process. The great majority of our cells are destined to die by just such a mechanism; relatively few die through injury or inability to sustain their own viability. In a human about a hundred thousand cells are produced every second by mitosis, and a similar number die by a physiological suicide process known as apoptosis. Most of the cells produced during mammalian embryonic development undergo physiological cell death before the end of the perinatal period. During our life span, over 99.9% of our cells undergo the same fate.