Methanotrophs are a group of bacteria that can use methane as their sole source of carbon and energy. They play a major role in carbon cycle and global warming by controlling emissions of methane, the second most important greenhouse gas following CO₂. In this review, we summarize recent progress on the physiology, phylogeny, and ecology of methanotrophs, with particular focus on the diversity of methanotrophic community in natural wetlands. The traditionally identified methanotrophs all belong to the phylum Proteobacteria. Based on intracytoplasmic membranes formation, predominant fatty acid types, the mechanism by which carbon is assimilated into biomass and phylogenetic characteristics, proteobacterial methanotrophs are divided into two groups, type Ⅰ and type Ⅱ (Gamma-and Alpha-proteobacteria, respectively). Up to now, 20 methanotrophic genera have been affiliated in phylum Proteobacteria, including two filamentous methanotrophs, Crenothrix polyspora and Clonothrix fusca. These two species have been characterized recently and form a new branch within the family Methylococcaceae. Verrucomicrobial methanotrophs, a remarkable new finding, are distantly related to the proteobacteria methanotrophs. They have been isolated from geothermal sites, seem to be restricted to extreme environments and form a new genus (Methylacidiphilum). Methanotrophs are also found in a novel phylum named NC10, which represents bacteria capable of aerobic methane oxidation coupled to denitrification under anoxic conditions. Two types of enzyme, a particulate methane monooxygenase (pMMO) and a soluble methane monooxygenase (sMMO) can be used by methanotrophs to execute the first step of methane oxidation. All known methanotrophs possess the pMMO, except genera Methylocella and Methyloferula which only have sMMO. Some methanotrophs of type Ⅰ and Ⅱ have both pMMO and sMMO. A different pMMO (pMMO2) is discovered in some type Ⅱ methanotrophs. pMMO2 has lower methane oxidation kinetics and enables these methanotrophs to consume methane at atmospheric concentrations. The pmoA and mmoX gene, encoding subunits of the pMMO and sMMO respectively, have been used as a functional marker for detecting methanotrophs in environmental samples. However, the current public pmoA sequences database is larger than that of the mmoX, and the sequence based pmoA phylogeny has good correlation to the 16S rRNA phylogeny. Facultative methanotrophs have been reported in the genera Methylocella, Methylocapsa, and Methylocystis. Some species of them can use compounds with carbon-carbon bonds as sole growth substrates, including acetate, large organic acids or ethanol. These findings broke the traditional notion that methanotrophs could only use one-carbon compounds, indicating that broader substrate utilization might be more common in methanotrophs. Methanotrophs have been isolated from various environments including habitats of extreme temperature, acidity or salinity. For example, some type Ⅰ methanotrophs (Methylocaldum, Methylococcus, and Methylothermus) were reported to have optimum growth temperatures above 40 ℃. On the other hand there are some methanotrophs (Methylobacter and Methylocella) adapted to cold environments and with optimum growth temperatures of 0-30 ℃. Some Methylacidiphilum species grow at extreme low pH of 2-2.5. But some Methylomicrobium species have the optimum pH of 9.0-9.5. Besides, some Methylomicrobium species and Methylohalobius crimeensis are halotolerant methanotrophs and have a growth optimum around 1-1.5 mol/L NaCl. In contrast, Methylocapsa KYG is very sensitive to NaCl and can only grow at low NaCl concentrations. By employing the 16S rRNA gene or functional genes as molecular markers, the methanotrophic communities have been extensively studied in many natural wetlands. A variety of molecular biological tools, such as T-RFLP, DGGE, FISH, clone library and pyrosequencing, have been used to detect the community diversity of methanotrophs in soils of these ecosystems. Most of the studies were conducted in acidic peat wetlands at the high latitudes of the Northern Hemisphere, especially in the United Kingdom and Russian. In these peatlands, most of the known methanotrophs belonged to type Ⅱ, such as genera Methylocystis, Methylocella and Methylocapsa. Especially genus Methylocystis, were widely distributed in acidic peatlands. Type Ⅰ methanotrophs, especially genus Methylobacter, were dominant in the cold Arctic wetlands in Norway and Finland. Furthermore, researches revealed that type Ⅰ and Ⅱ methanotrophs were widely present in natural wetlands in the United States and Japan. In 2012, the first study on methanotrophic diversity of wetland in the southern hemisphere was reported in Argentina. The high abundance of genus Methylocystis suggests that it is probably the major contributor to the methane oxidation in this Sphagnum wetland. Type Ⅰ and type Ⅱ methanotrophs were all detected and were present with different proportion in some natural wetlands of China. Great progress has already been made in the recent researches of the physiological and ecological characteristics of methanotrophs and their community diversity in the natural wetlands. With the arrival of the era of high-throughput sequencing and the development of new isolation and culture technology, the knowledge systems of methanotrophs will be refreshed more frequently.