It has been known for some time now that hypoxia is an important physiological parameter in tumor growth and response to therapy. The development and application of non invasive methods to determine the extent of tumor hypoxia as well as its modulation will improve cancer treatment strategies. Magnetic Resonance sequences that are based on the BOLD effect use the endogenous contrast agent deoxyhemoglobin as a source of contrast. This technique can be used to monitor the evolution of tumor oxygenation since there is a good correlation between the evolution of the partial pressure of oxygen (pO2) and the NMR parameters measured. The information provided by BOLD NMR is essentially qualitative in nature due to the complexity of the relationship between the pO2 and the NMR parameters measured. The factors at the origin of this complex relationship are discussed in this review. The advantages of the BOLD technique are non invasiveness and high spatial and temporal resolution. The method has been successfully applied in experimental and human tumors to monitor changes after respiratory challenges and pharmacological treatments. Additionally, the method has been used to provide maps of mature and functional tumor vessels and maps of spontaneous fluctuations of oxygenation and blood perfusion related to tumor acute hypoxia.