Hypoxia is a characteristic feature of many solid tumors which has been described in a wide range of tumor types. Its presence impairs the effectiveness of common anti-cancer therapies and accordingly, tumor hypoxia has been associated with an aggressive tumor phenotype, poor response to radio- and chemotherapy, and worse prognosis. In order to predict outcome and identify patients with a worse prognosis and/or patients that would benefit from appropriate treatments, in vivo measurement of tumor hypoxia is required. Given the difficulties associated with invasive methods, a non-invasive method is of major clinical interest. Although several candidate molecules have been labeled with PET and SPECT labels, none of them is used in daily clinical routine due to a number of difficulties that complicate their use. This review aims to give an overview of the most important hypoxia tracers, their prognostic significance and how these tracers can play a role in tomorrows personalized medicine.