Defects in apoptosis signaling pathways are common in cancer cells. Such defects may play an important role in tumor initiation because apoptosis normally eliminates cells with damaged DNA or dysregulated cell cycle, i.e., cells with increased malignant potential. Moreover, impaired apoptosis may enhance tumor progression and promote metastasis by enabling tumor cells to survive the transit in the bloodstream and to grow in ectopic tissue sites lacking the otherwise required survival factors. Finally, raised apoptosis threshold may have deleterious consequences by rendering cancer cells resistant to various forms of therapy. The intensive apoptosis research during the past decade has resulted in the identification of several proteins which may promote tumorigenesis by inhibiting apoptosis. Of special relevance in human cancer are those commonly expressed in primary tumors and functioning at the common part of the signaling pathway leading to apoptosis. Proteins fulfilling these criteria include antiapoptotic members of the Bcl-2 protein family, heat shock proteins, Hsp70 and Hsp27, as well as survivin, the novel cancer-associated member of the inhibitor of apoptosis protein family. Understanding the molecular mechanisms of action of these proteins may offer novel modes of rationally and selectively manipulating the sensitivity of cancer cells to therapy.