**Abstract**

This paper presents a classification and also an overview of fuel cells, including the working principles, the equations of the governing reactions, and the main applications. A brief exposure of thermodynamics and electrochemical theory describe the functioning of the fuel cells. Further, the proton exchange membrane fuel cells assembly, starting with the schematic presentation of the main components, the role of each component in fuel cell, the specific materials and their requested properties, and the way of assembling the components into device will be detailed. In conclusion, the challenges related to reliability and the cost and the targets for future development of the proton exchange membrane fuel cells for mobile and stationary applications will be presented.

**Keywords:** fuel cells, proton exchange membrane, catalyst layer, gas diffusion layer, membrane electrode assembly, bipolar plate
