The potential environmental benefits of using fuel cells in cars, buses and stationary combined heat and power (CHP) plants of different sizes have not been well-researched. This environmental analysis was conducted for the UK on a ＇full fuel cycle＇ basis, encompassing all greenhouse gas and regulated pollutant emissions for the supply chain and end-use technology under consideration. Solid polymer fuel cells (SPFCs) with methanol or natural gas reformers were analysed for cars, SPFCs and phosphoric acid fuel cells (PAFCs) with on-board hydrogen for buses. CHP plants were PAFCs or solid oxide fuel cells (SOFCs). Each option was compared with one or more conventional technologies. In all cases fuel cell technologies have substantially reduced emissions in comparison with conventional technologies. Regulated emissions are lowest, by up to two orders of magnitude, would those that do occur are primarily in the fuel supply chain. The fuel cell technologies are more efficient in, all eases, and carbon dioxide (CO2) emissions are reduced broadly in fine with energy savings. Methane emissions increase due to fuel switching, e.g. from petrol to natural gas powered buses, but from a very low base. The study pinpoints some areas in which alternative approaches could be made - the methods for generating and transporting hydrogen have a significant bearing on energy consumption and emissions. However, it is clear that from an overall emissions perspective tile use of fuel cells in transport and power generation is highly beneficial.