The concept that cell death can be a regulated process under molecular control is now well established. Much evidence also indicates that the molecular machinery for death has been well conserved through evolution. Until recently, efforts to identify components of this machinery were focused primarily on identifying endonucleases capable of cleaving DNA at internucleosomal sites, since this type of DNA degradation is a well-established characteristic of most, but not all, forms of apoptosis. However, observations from studies using enucleated cells as well as cell-free systems indicate that the central components of the cell death machinery, which we will call the executioner, are most likely localized to the cytosol. Furthermore, several lines of evidence indicate that proteases, particularly those of the emerging interleukin-lp (IL-lp)-converting enzyme (ICE) family, are good candidates for regulators or components (or both) of the executioner.