Prediction of response to therapy has been identified as an important tool to obtain a more customized treatment. It allows the selection of those patients who will benefit most from a particular therapy and prevents the exposure of patients to toxic, noneffective regimens. Recent technical advances and the introduction of novel markers in anatomical and functional imaging have created exciting opportunities for in vivo visualization and quantification of cell death. This review will focus on in vivo apoptosis imaging as a predictive marker for tumor response after radiation.