Electron Paramagnetic Resonance (EPR) spectroscopy is a very important technique for understanding radical reactions, electron transfer processes, and transition metal catalysis, which are all related...Electron Paramagnetic Resonance (EPR) spectroscopy is a very important technique for understanding radical reactions, electron transfer processes, and transition metal catalysis, which are all related to the ’reactivity of the unpaired electron’. These notes introduce the basics for applying EPR spectroscopy on reactive or catalytically active species as well as on spin probes.