Electron Paramagnetic Resonance (EPR) is the less-known, although elder, sister of NMR. What makes EPR unique is its ability to detect directly, to identify, and to quantify paramagnetic species, i.e. materials or compounds that have one or more unpaired electrons in the outer orbitals of their atoms or molecules.  EPR is a versatile tool for the study of materials and biological systems, especially if associated with other spectroscopic techniques, including its companion NMR. ​ A surprisingly large number of materials have unpaired electrons, including free radicals, transition metal ions, and localized defects in materials. EPR is applied, in vitro ed ex vivo, to the study of stable paramagnetic systems (stable radicals, antioxidants, metals, radicals derived by radiation- and photo-induced damage), of unstable paramagnetic systems (reactive oxidizing species and target-centred radicals) as well as of non-paramagnetic systems charged with paramagnetic probes (protein structures and membranes), with applications in health, medicine, pharmaceutical and food science industries, material science.