Cell death is often an active and highly choreographed process of cell suicide known as apoptosis. The molecular signals governing this process have been the subject of intense research in recent years. During apoptosis a profusion of pathways either promote cellular survival or lead to certain death. These pathways engage in constant crosstalk with those that control cell proliferation and other activities, in order to maintain the delicate balance between cell births and deaths. This commentary unit discusses two major pathways, active (antigen-induced) and passive (lymphokine-withdrawal) apoptosis, and provides background for the following unit on flow cytometry of apoptosis.