This review evaluates the potential of electron paramagnetic resonance (EPR, also known as electron spin resonance, ESR) as an in situ technique for analyzing structure-reactivity relationships in heterogeneous catalysis under reaction condition. It starts with a general overview on the state of the art, followed by a short introduction of its theory. In due course the use of the method is illustrated by three selected application examples. At the end a critical evaluation of opportunities and limitations of in situ EPR in heterogeneous catalysis is given.