Proton Exchange Membrane (PEM) water electrolysis system is considered to be a promising technology to generate green hydrogen from renewable energy sources. The performance of PEM depends on several conceptual and operational parameters, such as temperature, water content and thickness of membrane, the pressure of anode and cathode regions, and cathode and anode exchange current density. Identifying these parameters and their effects on the efficiency of PEM could help researchers to improve the performance of PEM by increasing safety, the quantity and hydrogen output, and minimizing the cost of hydrogen generation. The PEM efficiency and the effects of the main parameters that influence the performance of PEM are presented and reviewed in this paper.