Nitroxides (aminoxyl radicals) are a class of stable organic radicals where the unpaired electron is stabilized by steric shielding and delocalisation over the N-O bond. The extremely long lifetimes of nitroxide radicals render them suitable for various applications in spectroscopy, in particular for investigations in structural biology. Nitroxides have gained a unique place within electron paramagnetic resonance (EPR) spectroscopy as spin labels that can reveal dynamic and structural information in biomolecules. Through a process known as site-directed spin labelling (SDSL), nitroxides can be covalently attached to the biomolecule of interest, which is then analysed by EPR spectroscopy. This approach is especially powerful when integrated with the double electron-electron resonance (DEER) experiment (also known as pulsed electron-electron double resonance, PELDOR), which can extract the interspin distance and relative orientation of two paramagnetic centres within a biomolecule or biomolecular complex. Nitroxides also hold a firmly established position as polarising agents for sensitivity enhancement by dynamic nuclear polarisation (DNP) in nuclear magnetic resonance (NMR) spectroscopy, a technique that is of particular utility in the study of large biomolecules. This ability to transfer spin polarisation to the surrounding medium has, moreover, led to the development of nitroxides as promising metal-free contrast agents for MRI imaging. In order to be suitable for these numerous applications in spectroscopy and imaging, nitroxide derivatives should ideally exhibit robustness towards reduction, possess optimal electron spin relaxation times, and be adequately soluble in aqueous environments. The need for stability towards reduction arises from a tendency for nitroxides to be converted to diamagnetic hydroxylamines in reducing media, such as the intracellular environment. Several factors determine the reductive stability of nitroxides, including the presence of charges, electron-donating or -withdrawing functional groups, the size of the nitroxide ring, and the steric bulkiness of the substituents at the quaternary αpositions next to the paramagnetic centre (Figure ). Saturated five-membered pyrrolidinyl nitroxides are remarkably stable when compared to unsaturated five-membered pyrrolinyl and the least stable six-membered piperidinyl nitroxides (Figure , top). Tetraethyl substituents surrounding the nitroxide impart increased steric shielding, thereby contributing to exceptional stability compared to their tetramethyl counterparts (Figure , bottom).