For a clean environment, low-temperature fuel cells are particularly important to power familiar devices, such as portable electronics (cell phones, computers, cam recorders, etc.) or electrical vehicles (buses, trucks, and individual cars). Among them, the alkaline fuel cell working at 80 °C with pure hydrogen, the proton exchange membrane fuel cell that can operate at temperatures ranging from ambient to 70–80 °C with hydrogen either produced by water electrolysis or by hydrocarbon reforming, and the direct alcohol fuel cell that realizes at higher temperatures (up to 120–150 °C) the direct electrooxidation of methanol or ethanol are particularly convenient. In these fuel cells, because of the relatively low working temperatures, the kinetics of the electrochemical reactions involved (fuel oxidation and oxygen reduction) is rather slow. Therefore, to improve the reaction kinetics, by a careful design of the electrode catalyst, it is necessary to determine detailed reactionmechanisms, where all the adsorbed species and intermediate products have been clearly identified. The use of purely electrochemical techniques is not at all sufficient to do it, and electrochemical methods have to be coupled with spectroscopic methods (infrared spectroscopy, mass spectroscopy, etc.) and analytical methods (gas chromatography, high-pressure liquid chromatography, radiotracers, etc.) in order to identify the different species involved and to evaluate their concentration or surface amount. After the establishment of the reaction mechanism, particularly the knowledge of the rate determining step, it is important to design suitable electrocatalysts able to activate preferentially the rate determining step.The various methods to prepare such catalysts are first presented. Then, the different physicochemical methods used to evaluate their properties and to determine the reaction mechanisms are discussed. Finally, the reaction mechanisms of the most important electrochemical reactions involved in low-temperature fuel cells, that is, the electrooxidation of hydrogen, carbon monoxide, methanol, and ethanol and the electroreduction of oxygen, have been established.