Presently, free radicals and oxidants are considered to mediate from signaling circuits involved in physiology and pathology to cell and tissue injury. The elucidation of these many inter-related processes requires a better understanding of cellular oxidative mechanisms many of which are mediated by protein radicals. Here, we will discuss the potentialities of EPR spin-trapping of protein radicals to unravel oxidative mechanisms. An overview of the methodology and its application to identify protein residues that are the target of specific oxidants, characterize emerging oxidants, and discriminate radical from non radical mechanisms will be presented. The examples are based on work developed in our laboratories but will be discussed in a broad scenario to emphasize that simple experiments can provide relevant insights into the biological reactivity of known and emerging biological oxidants and into signaling mechanisms.