Copper-containing enzymes that react with O 2 play a key role in many biological processes. Mononuclear, dinuclear and trinuclear copper centers function in O 2 binding, activation and subsequent substrate oxidation. Recent advances in the structural biology of O 2-activating copper enzymes range from the identification of novel copper centers, such as that of particulate methane monooxygenase, to the elucidation of the details of O 2 binding and reactivity in peptidylglycine α-hydroxylating monooxygenase. Structures of phenoxazinone synthase and Fet3 contribute to our understanding of multicopper oxidases. Additionally, details of the tyrosinase structure provide new insight into how dicopper sites confer substrate specificity. A common theme for each of these enzymes is that the protein scaffold plays a major role in dictating the overall function.