The term βhigh-valentβ refers to iron complexes of porphyrins and related macrocycles in which the oxidation state of the iron center exceeds III. High-valent iron porphyrins and chlorins are important biological transients whose intermediacy has been demonstrated in numerous peroxidase and catalase enzymes. Two species, compounds I and II, are spectroscopically detectable upon stoichiometric addition of monooxygen donors to resting ferric enzymes. Compounds I and II are formally two and one oxidizing equivalents respectively above the ferric state. In compound II the oxidizing equivalent has been shown by spectroscopic studies to be located on iron as an oxoiron(IV) unit. The spectroscopic and magnetic properties of compound I support the structural assignment of an S = 1 oxoiron(IV) unit magnetically coupled to a heme Ο-cation radical (S = 1/2). Studies on model hemes have contributed much to the understanding of protein chemistry. Much work has been accomplished with meso-tetaarylporphyrins and, more recently, with physiologically congruent meso-unsubstituted pyrrole Ξ²-substituted complexes. Compounds I of both proteins and synthetic models have been characterized by a wide array of spectroscopic methods, including UV-vis, NMR, resonance Raman, EPR, variable-temperature/variable-field magnetic MΓΆssbauer, magnetic circular dichroism and extended X-ray absorption fine structure spectroscopy. Results of these studies are summarized. Recent developments, which promise to yield a detailed picture of electronic structure, are variable-temperature magnetic circular dichroism, studies in the pre-K-edge region and L-edge X-ray absorption spectroscopy. Time-resolved X-ray diffraction techniques have been applied to obtain the first structural data on the protein forms of compound I.