The modern chemical industry relies heavily on homogeneous and heterogeneous catalysts. Understanding the operational mode, or reactivity, of these catalysts is crucial for improved developments and enhanced performance. As a result, various spectroscopic techniques are inevitably used to characterize and interrogate the mechanistic details of the catalytic cycle. Where paramagnetic centres are involved, ranging from transition metal ions to defects and radicals, EPR spectroscopy is without doubt the technique of choice. In this review we will demonstrate the wealth and breadth of information that can be gleaned from this technique, in the characterization of homogenous and heterogeneous systems of catalytic importance, whilst illustrating the advantages that modern high-field and pulsed EPR methodologies can offer.