Caspases are a family of cysteine proteases, which are phylogenetically conserved throughout metazoans and serve many different roles. Mammalian caspases are categorized into two functional groups, those involved in immunity and those that facilitate apoptotic cell death. Caspases are expressed as inactive precursors inside cells and must be activated to cleave specific target substrates. Pro-apoptotic caspases cleave a larger subset of intracellular proteins to mediate cell suicide by apoptosis, while caspases involved in immunity can kill cells by alternative non-apoptotic mechanisms. Pro-apoptotic caspases are essential for embryonic development and for normal tissue homeostasis.