Hypoxia has been observed in a variety of human tumor types and evaluating tumor hypoxia is important because it increases resistance to radiotherapy and chemotherapy by inducing proteomic change that allow the tumor cell to survive in their hypoxic environment. One of the major proteomic changes is HIF-1 expression, and HIF-1 has become a target for anti-cancer drugs development because of its central role in hypoxia-mediated aggressiveness of tumor cells and their resistance to therapy. Since tumor hypoxia is a key mechanism that leads to resistance of treatment, a large number of challenges for hypoxia imaging including magnetic resonance, optical, and nuclear imaging have been reported. These hypoxia imaging techniques may have potential in selecting cancer patients who would benefit from treatments that overcome the presence of hypoxia. Hypoxia imaging could also be used to document whether or not and the extent to which reoxygenation of tumors occurs during cancer treatment. One of key requirements of ideal method for imaging hypoxia is that the method should be non-invasive. From an imaging perspective, PET is also one of leading tools for imaging hypoxia because of its high spatial resolution, high sensitivity, and advantages for visualizing molecular events in living human tissue. In this review, PET-based radiopharmaceuticals including (18)F-FMISO, (18)F-FETNIM, (18)F-FAZA, and radioactive Cu-ATSM were summarized from published studies about radiosyntheses, pre-clinical data, and clinical data, which are the lead contenders for human application.