The majority of catalytic reactions involves reduction and/or reoxidation steps in which electrons are transferred between catalysts and substrates. EPR spectroscopy can sensitively probe the local environment of paramagnetic catalytic sites as well as their behavior in catalytic redox processes since it can be applied under a wide range of conditions. After a short summary of the most important application examples of in situ EPR in redox catalysis, the main features of EPR spectra such g and A matrices and approaches of their evaluation are presented using model systems that contain Au atoms deposited on MgO single crystal surfaces. To illustrate the versatility of in situ EPR for deriving structurereactivity relationships in catalysis, two application examples are presented in more detail:(1) Analysis of heterogeneous supported VO x /TiO 2 catalysts during oxidative dehydrogenation of propane in the gas phase and (2) Study of photocatalytic water splitting over a homogeneous catalytic system comprising an iridium photosensitizer complex and an iron carbonyl catalyst.