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EPR spectroscopy is a magnetic resonance technique that detects species carrying one or more unpaired electrons: organic radicals, transition-metal ions and point defects in solids. The sample is placed in a static magnetic field and irradiated at microwave frequency; the electron spin is excited when the field and the frequency meet the resonance condition, and the g value, the hyperfine splitting from neighbouring nuclei and the zero-field splitting report on the electronic structure and on the environment of the spin ^[1]. Sensitivity is high compared with nuclear magnetic resonance, and because only paramagnetic centres are visible the method is selective for radicals embedded in a diamagnetic matrix ^[2]. Continuous-wave instruments record the absorption while the field is swept, whereas pulsed experiments separate the interactions in time; double electron-electron resonance measures the dipolar coupling between two spins and so gives distances of roughly 1.5 to 8 nanometres, a range that has been used to follow conformational change in proteins ^[3], and a distance distribution can still be recovered when the zero-field splitting is large ^[4]. Assigning a spectrum generally requires simulating it, and g tensors and hyperfine couplings are now also obtained from first-principles calculation ^[5].