Concentrations of atmospheric methane have more than tripled since pre-industrial times, argua- bly the largest global change in atmospheric composition caused by human beings. This talk re- views the evidence for this change, and examines the implications for atmospheric chemistry and climate. Data from polar ice cores show that methane concentrations were low during glacial pe- riods (~400 parts per billion) and high during interglacial intervals (600 - 700 parts per billion). In- creases beyond values form recent earth history began in the 18th century, leveled off abruptly between 1990 and 2005, and then started to rise sharply again. We examine some current re- search into the process es responsible for human-related emissions of methane to the atmos- phere: changes in Arctic soils, agriculture, and natural gas infrastructure.