Today, one of the most serious problems in the world is reliance upon the combustion of fossil fuels, because they are limited in supply and there is a large demand for them. This leads to continuous increase of their cost; also, they are located in different regions of the world, and this may lead to local conflicts. Moreover, their combustion is dangerous to the health of many species, because pollution is an indirect cause of global warming. One of the possible solutions, which combines both environmental and economical priorities of our world, is the research and development of a power source that has low pollutant emissions, is energy efficient, and has an unlimited supply of fuel for a growing world population. According to these problems, some alternative energy technologies have been researched and developed, such as solar, wind, hydroelectric power, bio-energy, geothermal energy and many others. Each of these alternative energy sources has its advantages and disadvantages, and obviously they are in different stages of development. In this situation, fuel cells are identified as one of the most promising technologies. If we want to do a comparison between fuel cells and the other types of renewable energy technologies, we can say that fuel cells have a lot of advantages over the others. For example, they can use a variety of different fuels to generate power (e.g. hydrogen, methanol, fossil fuels to biomass-derived materials, etc.) and they can also provide electricity by working with large power plants to become more decentralized and increase efficiency. In fact, they have one of the highest efficiencies between the competing energy conversion devices. Today, fuel cells have reached a level of development from which it is possible to predict they can enter in a lot of markets in which electricity must be produced with high efficiency and low environmental impact and, as we will see in the next chapters, they can be employed in a large range of different applications. Moreover, numerous fuel cell-based power plants have been built and operated successfully, on a scale from both tens of megawatt to tens or hundreds of kilowatts and more, and many small fuel cell units are in use with output between a few milliwatts and a few watts. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved..