Recent evidence has highlighted the pivotal role of intracellular cystine proteases (caspases) and mitochondrial-derived apoptogenic factors, such as cytochrome C and AIF, during apoptotic execution. We hypothesize that if the interplay between caspases and mitochondria determine the fate of tumor cells during chemotherapy, then it is logical to identify compounds that directly target these effector components of the death pathway. In this regard, we have shown that the cytosolic release of cytochrome C could occur in the absence of a drop in the transmembrane potential of the mitochondria and without the induction of mitochondrial permeability transition. Secondly, we demonstrated that the mere release of cytochrome C in the absence of efficient caspase activation was not sufficient for effective induction of drug-induced apoptosis. We identified mitochondrial hydrogen peroxide, in addition to cytochrome C release, and downstream induction of cytosolic acidification as critical effector mechanism(s) that determine the efficacy of anti-cancer therapy. These findings have substantially added to our knowledge of the signaling mechanism(s) operative in tumor cells, in particular the role of mitochondria in the execution phase.