Apoptosis is a regulated form of cell demise that can be induced or blocked by groups of specific stimuli. Occurring in all living tissues, it is thought critical to the maintenance of homeostasis and is implicated in lowering susceptibility to tumour growth. Conversely, over-sensitivity to apoptotic triggers can cause cells to be lost inappropriately from tissues, as appears to occur in neurodegenerative diseases. This book examines the molecular pathways regulating apoptosis, progressing from the nematode worm through Drosphila to the more complex pathways evident in mammalian cells. Differences in the cell death pathway within specific tissues are addressed, as are the different genes that may act to regulate progressive steps.