Nitroxides (also known as aminoxyl or nitroxyl radicals) are suited for innumerable applications across the chemical sciences. These stable organic radicals are, for example, used as oxidants in synthetic organic chemistry, mediators for radical polymerization, redox-active components for materials science, and as potential therapeutic agents. Nitroxides have also found a unique place of significance in spectroscopy. Due to the localized radical distribution, small molecular size, and readily tunable properties through structural modification, nitroxides are highly suitable for use as observable probes in electron paramagnetic resonance (EPR) spectroscopy and EPR imaging. Moreover, nitroxides are used in nuclear magnetic resonance (NMR) spectroscopy as dynamic nuclear polarization (DNP) transfer agents, paramagnetic relaxation enhancement (PRE) probes, and as contrast agents in magnetic resonance imaging (MRI). Within the field of EPR spectroscopy, double electronelectron resonance spectroscopy (DEER), also known as pulsed electron-electron double resonance (PELDOR), is a powerful tool for the investigation of biomolecular structure. DEER experiments enable the measurement of nanometer-scale distances and angles between two spin centers (referred to as spin labels), which have been incorporated into biomolecular systems via site-directed spin labelling (SDSL). The resolution of DEER measurements is among other things influenced by the temperature-dependent relaxation parameter Tm, the phase memory time, which should be as high as possible. For imaging and for modern spectroscopy applications, moreover, nitroxides should possess a sufficiently high aqueous solubility and stability towards the reducing environments typically found within cells. In the presence of reducing agents such as ascorbate or glutathione, nitroxides are known to undergo reduction to diamagnetic hydroxylamines, which are not detectable by EPR. Several structural factors influence nitroxide stability towards reduction (Figure ). The size and saturation of the nitroxide ring has a significant impact: saturated five-membered pyrrolidinyl nitroxides are more stable than the unsaturated five-membered pyrrolinyl nitroxides, while six-membered piperidinyl nitroxides are more prone to reduction. Electronic factors also influence the reduction resistance of nitroxides: electron-donating substituents enhance stability, while electron-withdrawing moieties decrease it. Finally, the α-substituents shielding the paramagnetic center are a crucial factor. Replacing the most common methyl substituents with spirocyclohexyl groups result in a slight increase of the stability in reducing environments. However, stability is significantly enhanced by more conformationally mobile ethyl substituents, which effectively occupy a larger spatial volume surrounding the spin center. Beyond their impact on reduction stability, α-substituents also have a critical influence on Tm. The rotation of methyl groups is a significant contributor to the relaxation of nitroxides at temperatures, especially above 70 K. Thus, the commonly used tetramethyl-substituted nitroxides require the use of cryogenic temperatures for DEER. This effect is also present in tetraethyl-substituted nitroxides, where a lower energy barrier for methyl rotation leads to increased relaxation rates at even lower temperatures. In spirocyclic systems, however, the lack of methyl groups significantly enhance Tm, even at higher temperatures. Thus, the design of nitroxides for in-cell applications is often a compromise between high reduction stability and long Tm.