Peroxidases are a class of heme enzymes that catalyze the oxidation of a wide variety of organic and inorganic compounds at the expense of hydrogen peroxide. The catalytic cycles of peroxidases start from the reaction of the ferric resting state with hydrogen peroxide, producing a reactive oxoiron(iv) porphyrin π-cation radical species known as compound I via a ferric hydroperoxide intermediate named as compound 0, which is not detected in normal conditions. Then, compound I induces the first single-electron oxidation of a substrate to form an oxoiron(iv) porphyrin species know as compound II, which returns to the ferric resting state by the second single-electron oxidation of the substrate. These intermediates are key compounds in understanding the chemistry and biology of heme peroxidases. In addition, the oxoiron(iv) porphyrin π-cation radical species is also known as a reactive intermediate not only for peroxidases, but also for cytochrome P450, catalase and synthetic metal porphyrin catalysts. Therefore, there have been studies on the electronic structures, reactivity, and reaction selectivity of these intermediates with synthetic peroxidase model complexes. This chapter concentrates on the electronic structure and reactivity of heme model complexes of the ferric resting state, compound 0, compound I and compound II. The model reactions and the model complexes for the ferric hypochlorite intermediate of heme haloperoxidases are also introduced.