The author of this book has been contributing to developments of an electron paramagnetic resonance (EPR) imaging system and its applications for more than 10 years. This system was designed for EPR measurements of experimental animals such as rats and mice, and its applications were mainly for biological researches. Thus, a lot of parameters to design an EPR imaging system are determined on the assumption that this system can be used for in vivo studies. The main aim of performing in vivo EPR measurements is to estimate in vivo reducing ability that cannot be obtained by other measuring methods at present. In this book, before main chapters about EPR imaging, this aim and specifications of apparatuses for achieving this aim are stated as a background. After description of instrumental components of an in vivo spectrometer, a theory, instruments, data processing, and applications of EPR imaging are explained. In the chapters about instruments and data processing, their details are described so that one can build the apparatus and obtain data. In the chapter about applications, examples of in vivo estimation of reducing ability in experimental models of some diseases are presented. In the last chapter, an explanation about region-selected intensity determination (RSID), a novel method that allows estimation of in vivo reducing ability at a selected anatomical region without complications of previous EPR imaging method, is stated.