Despite major improvements in the surgical management the prognosis for patients bearing malignant gliomas is still dismal. Malignant gliomas are notoriously resistant to treatment and the survival time of patients is between 3-8 years for low-grade and anaplastic gliomas and 6 - 12 month for glioblastoma. Increasing malignancy of gliomas correlates with an increase in cellularity and a poorly organized tumour vasculature leading to insufficient blood supply, hypoxic areas and ultimately to the formation of necrosis, a characteristic of glioblastoma. Hypoxic/necrotic tumours are more resistant to chemotherapy and radiation. Hypoxia induces either directly or indirectly (through the activation of transcription factors) changes in the biology of a tumour and its microenvironment leading to increased aggressiveness and tumour resistance to chemotherapy and radiation. This review is focused on hypoxia-induced molecular changes affecting glioma biology and therapy.