Peroxidases catalyze the reduction of peroxides and the oxidation of various physiologically important molecules iin a multi-step reaction. The enzyme first reacts with peroxide to give Compound I. In Compound I the iron atom has been oxidized and an organic group, R, also is oxidized to a free radical. R is either the porphyrin ring of the heme or, as in cytochrome c peroxidase (CCP), a Trp side chain. Peroxidases have been especially important in understanding the nature of protein/porphyrin centered radical and the nature of the Fe(IV)=O intermediate primarily because these species are quite stable in peroxidase despite the high redox potentials. As a result a variety of biophysical methods, spectroscopy, and crystallography has been applied to peroxidases in an attempt to understand why some peroxidases form a porphyrin radical while others form an amino acid radical. Crystallography, however, presents special challenges since Compounds I and II of peroxidases are both readily reduced in the x-ray beam. However, using composite data collection methods, robotic data collection at synchrotron x-ray sources, and single crystal spectroscopy it has been possible to elucidate the structural details of CCP Compound I.