Necrosis and apoptosis are two ways by which cells die. A major concept of apoptosis is that it is a controlled process. From this concept it follows that cells contain a molecule or molecules which under specific, regulated circumstances mediate cell death. Recent data confirm that oxygen free radicals can be mediators of apoptosis. Chemicals could induce apoptosis due to reactive oxygen species production and changes in the intracellular redox state. Therefore, a complete understanding of the processes involved in apoptosis, and mechanisms of its manipulation, could provide novel strategies to the control of xenobiotic toxicity and give an impetus to design new therapeutic interventions.