Despite multimodal treatment malignant gliomas still remain incurable. Molecular markers represent a useful tool for the development of tailored therapeutic options. In gliomas the transcription factor HIF-1α, a key mediator of hypoxic response, is a poor prognostic marker. In contrast, mutations in the IDH enzymes are associated with better outcome for patients with glioma. The aim of the present thesis was to investigate the influence of HIF-1α-Inhibition and expression of mutant IDH1 on cell biological and radiobiological behavior of malignant glioma cell lines. Targeting HIF-1α attenuated the hypoxia-induced radioresistance of the investigated glioma cell lines. Furthermore, overexpression of mutant IDH1 caused a less aggressive phenotype and was directly linked to an increased sensitivity to radiotherapy of glioma cells independent of the oxygen conditions. In conclusion, inhibition of HIF-1α is a promising strategy to sensitize hypoxic gliomas to radiotherapy and detection of the genetic status of IDH1 before radiotherapy might help to identify patients with a radioresistant IDH1 wild type glioma who should receive more aggressive treatments.