Methane is a more potent greenhouse gas with nearly 23-times more heat trapping capacity than carbon dioxide and is a major contributor to global warming and climate change. There is a constant search for technologies that can remove this greenhouse gas from the environment. This chapter describes methane-oxidizing enzymes, e.g., soluble methane monooxygenase (sMMO) and particulate methane monooxygenase (pMMO), and their modes of action toward methane and other pollutants. It reviews current progress on overall architecture, active sites, and substrate reactivities of MMO, and it describes the delivery of methane, oxygen, and electrons to its active site, with an emphasis on the roles of methaneoxidizers in methane regulation in the environment. Readers will learn about modeling the structures of MMO sequences and molecular docking using methane. Results of mutations in MMOs and their regulation of activity are also discussed. Finally, applications of MMOs, as well as methanotrophs and methylotrophs in biofuel production using methane as a feedstock, are discussed.