Hydra is one of the simplest metazoans and thus an important model organism for studies on the evolution of developmental mechanisms in multi-cellular animals. In Hydra apoptosis is involved in the regulation of cell numbers in response to feeding, in regeneration and in the removal of non-self cells. It also participates in the maintenance of cellular homeostasis in germ cells. During oogenesis a special "arrested" apoptosis of nurse cells is observed. The morphology of apoptotic hydra cells is almost indistinguishable from apoptosis in higher animals and caspases as well as members of the Bcl-2 family participate in the process.