Hypoxia, a characteristic of most human solid tumors, is a major obstacle to successful radiotherapy. Aim of this PhD study was to define the role of the Bcl 2 protein family in radioresistance conferred by exposure to acute and adaptation to cycling hypoxia and to evaluate whether pharmacologic targeting of the Bcl 2 rheostat improves sensitivity of hypoxic and hypoxia adapted cancer cells to the cytotoxic action of radiotherapy. The results demonstrated that exposure to acute and adaptation to cycling hypoxia (represented by hypoxia selected NCI H460 cell model) alters the balance of Bcl 2 family proteins in favor of anti apoptotic family members, thereby elevating the apoptotic threshold and attenuating the success of radiotherapy. It was further shown that blocking anti apoptotic Bcl 2 family members by the clinically relevant BH3 mimetic ABT 263 enhanced the sensitivity of HCT116 colon cancer and NCI H460 lung cancer cells to the cytotoxic action of ionizing radiation. Importantly, this effect was not only observed in normoxia, but to a similar or even higher extent in severe hypoxia. ABT 263 furthermore enhanced the response of xenograft tumors of non selected and hypoxia selected NCI H460 cells to radiotherapy, thereby confirming the beneficial effect of combined treatment in vivo. Targeting the Bcl 2 rheostat with ABT 263 therefore is a particularly promising approach to overcome radioresistance of hypoxic and hypoxia adapted cancer cells. Moreover, intrinsic as well as ABT-263- and irradiation induced regulation of Bcl 2 family members was found to determine therapy sensitivity. In this context, Mcl 1 was identified as a resistance factor that interfered with apoptosis induction by ABT 263, ionizing radiation, and combinatorial administration. Finally, hypoxia selected NCI H460 cells were characterized by an elevated capability to enhance mitochondrial respiration, which depended on fusion protein Opa1. This represents an additional mechanism of adaptation to adverse hypoxic conditions that might be relevant for improved survival and therapy resistance of cancer cells adapted to cycling hypoxia. Collectively, the findings of the present PhD thesis provide novel insights into the molecular determinants of hypoxia mediated resistance to apoptosis and radiotherapy and a rationale for future therapies of hypoxic and hypoxia adapted tumor cell fractions.