This chapter explores the role of liquefied natural gas (LNG) in global energy systems, its environmental impact, transportation issues, and potential as a marine fuel. LNG is a fossil fuel consisting primarily of methane, with smaller amounts of ethane, propane, butane, nitrogen, and carbon dioxide. A large LNG carrier is typically used because LNG's high density compared to NG makes it economical for long-distance transport, but it has the operational issues of boil-off gas (BOG) generation due to the very low storage temperature of LNG. With stricter GHG emission regulations, advanced dual-fuel gas injection engines with reliquefaction are now being used. While LNG emits less CO2 than conventional heavy fuel oil (HFO), its GHG emissions intensity may close to that of HFO when considering engine type and methane slip. Demand for LNG as a fuel will gradually decline in the distant future because it is essentially a fossil fuel, but the use of LNG as a feedstock for alternative fuels such as hydrogen is expected to continue for a considerable period of time. In addition, alternative sources of LNG, such as biogas, can offer a more sustainable future by reducing net GHG emissions.