The first proapoptotic caspase, CED-3, was cloned from Caenorhabditis elegans in 
1993 and shown to be essential for the developmental death of all somatic cells. 
Following the discovery of CED-3, caspases have been cloned from several 
vertebrate and invertebrate species. As reviewed in other articles in this issue 
of Cell Death and Differentiation, many caspases function in nonapoptotic 
pathways. However, as is clear from the worm studies, the evolutionarily 
conserved role of caspases is to execute programmed cell death. In this article, 
I will specifically focus on caspases that function primarily in cell death 
execution. In particular, the physiological function of caspases in apoptosis is 
discussed using examples from the worm, fly and mammals.