A fuel cell is an electrochemical converter which continuously converts the chemical energy from a fuel and an oxidant into electrical energy, heat and other reaction products. This chapter deals exclusively with hydrogen fuel cells and focuses on two types of cells: proton exchange membrane fuel cells (PEMFCs) and solid oxide fuel cells (SOFCs). It discusses various types of classifications based on the operation of the electrolyte, the basic of acid/basic medium, operating temperature, and type of electrolyte. A fuel cell stack has to be surrounded with actuators and devices which are necessary for its operation. The ensemble is called a fuel-cell system. The potential applications for fuel cells are very broad. These applications may be described in three categories: mobile applications, stationary applications and transport applications. Controlled Vocabulary Terms electrochemical analysis; fuel cells; proton exchange membrane fuel cells; solid oxide fuel cells