Accumulative magnetic circular dichroism (MCD) spectra have shown that MCD is indispensable in exploring the structures and electronic states of hemoproteins. Theoretical and instrumental introductions of MCD spectroscopy were followed by a specific theoretical treatment for applying MCD to various oxidation, ligand, and spin states of hemoproteins. By including explicit electronic states of the heme iron, the theory can systematically interpret the MCD results of various states of hemoproteins. The MCD spectra of myoglobin derivatives offered a typical example for various states of hemoproteins. As an MCD application to heme enzymes, MCD studies on cytochrome P-450 were described in detail, and other extensive applications of MCD to various hemoproteins were cited.