Electron spin resonance (ESR) is a widely employed spectroscopic technique for studying systems with unpaired electron spins, such as molecular radicals. Typically, many billions of spins are required to get a detectable ESR signal, which is subject to extensive ensemble averaging. Downscaling ESR to a single molecule allows studying the signatures of each individual molecule separately, applicable to biomolecules in their native environment, for example. Single-molecule ESR offers several novel research avenues, such as in quantum sensing with a single molecule. Over the last decades, four different single-molecule ESR approaches have been developed, which rely on either optically detected magnetic resonance or scanning-probe microscopy. An introduction into these four approaches including their deployment in pioneering works will be provided.