This chapter discusses the metabolic aspects of aerobic obligate methanotrophy. Aerobic methanotrophs are a unique group of gram-negative bacteria that use methane as carbon and energy source. Methanotrophs have been studied intensively over the past 40 years since these bacteria possess significant metabolic potential for practical use in the biotransformation of a variety of organic substrates, bioremediation of pollutants the production of single-cell protein (SCP), and value-added products. They also play a vital role in the global methane cycle, mitigating the emissions and green-house effects of methane on the Earth's climate. Methanotrophs build all of their cell constituents from C1 compounds by employing special biosynthetic pathways for phosphotrioses, which are different from those of heterotrophic bacteria.