Amino acid radicals are found as redox active reaction intermediates in an increasing number of enzymes. It has been demonstrated that EPR and its related techniques are important tools for identification, e.g. discrimination of tyrosyl and tryptophan radicals, investigation of structure and protein interactions, and even site selective assignment of freeze-trapped protein-based radicals. The combined use of EPR and theoretical calculations is a valuable tool in elucidating the catalytic mechanism of oxidative enzymes, mainly peroxidases, laccases and their variants. These studies are of great help in modelling new robust systems for industrial applications.