Particulate methane monooxygenase (pMMO) is a three-subunit integral membrane enzyme that catalyzes the oxidation of methane to methanol. Although pMMO is the predominant methane oxidation catalyst in nature, it has proved difficult to isolate, and most questions regarding its molecular structure, active site composition, chemical mechanism, and genetic regulation remain unanswered. Copper ions are believed to play a key role in both pMMO regulation and catalysis, and there is some evidence that the enzyme contains iron as well. A number of research groups have solubilized and purified or partially purified pMMO. These preparations have been characterized by biochemical and biophysical methods. In addition, aspects of methane monooxygenase gene regulation and copper accumulation in methanotrophs have been studied. This review summarizes for the first time the often controversial pMMO literature, focusing on recent progress and highlighting unresolved issues.