Electron paramagnetic resonance has played a crucial role in the determination of the properties and in explaining the origin of the peculiar behaviour of molecular nanomagnets, i.e., the slow relaxation of the magnetization at low temperature and the observation of quantum tunnelling of the magnetization. This is due to its ability to probe directly the spin levels rather than providing thermodynamic averages as in standard magnetic measurements. In particular, thanks to recent technical improvements in both reliability and sensitivity, high-field high-frequency EPR has gained widespread use for the characterization of these systems. We will briefly review here the different aspects underlying the use of this technique, highlighting some results of general interest obtained in the last decade on different class of magnetic clusters.