Cytochrome P450 (P450) enzymes catalyze a variety of oxidation and some reduction reactions, collectively involving thousands of substrates. A general chemical mechanism can be used to rationalize most of the oxidations and involves a perfenyl intermediate (FeO3+) and odd-electron chemistry, i.e. abstraction of a hydrogen atom or electron followed by oxygen rebound and sometimes rearrangement. This general mechanism can explain carbon hydroxylation, heteroatom oxygenation and dealkylation, epoxidation, desaturation, heme destruction, and other reactions. Another approach to understanding catalysis involves analysis of the more general catalytic cycle, including substrate specificity, because complex patterns of cooperativity are observed with several P450s. Some of the complexity is due to slow conformational changes in the proteins that occur on the same timescale as other steps.