Peatlands are wetland ecosystems that accumulate dead organic matter (i.e., peat) when plant litter production outpaces peat decay, usually under conditions of frequent or continuous waterlogging. Collectively, global peatlands store vast amounts of carbon (C), equaling if not exceeding the amount of C in the Earth's vegetation; they also encompass a remarkable diversity of forms, from the frozen palsa mires of the northern subarctic to the lush swamp forests of the tropics, each with their own characteristic range of fauna and flora. In this review we explain what peatlands are, how they form, and the contribution that peatland science can make to our understanding of global change. We explore the variety in formation, shape, vegetation type, and chemistry of peatlands across the globe and stress the fundamental features that are common to all peat-forming ecosystems. We consider the impacts that past, present, and future environmental changes, including anthropogenic disturbances, have had and will have on peatland systems, particularly in terms of their important roles in C storage and the provision of ecosystem services. The most widespread uses of peatlands today are for forestry and agriculture, both of which require drainage that results in globally significant emissions of carbon dioxide (CO2), a greenhouse gas (GHG). Climatic drying and drainage also increase the risk of peat fires, which are a further source of GHG emissions [CO2 and methane (CH4)] to the atmosphere, as well as causing negative human health and socioeconomic impacts. We conclude our review by explaining the roles that paleoecological, experimental, and modeling studies can play in allowing us to build a more secure understanding of how peatlands function, how they will respond to future climate- and land-management-related disturbances, and how best we can improve their resilience in a changing world.