The necessity for a better understanding of the basic processes that determine the operation of fuel cells became evident during the devel­ opment of practical units in the last three decades. The search for efficient electrocatalysts in low-temperature fuel cells intensified the general study of the nature and the role of the electrode material. Re­ search on the complex mechanisms of the anodic oxidation of different fuels and of the reduction of molecular oxygen on solid electrodes was stimulated, and the strong influence of adsorbed species on the electrode reaction in question was investigated. Suitable electrolytes had to be found for the high-temperature fuel cells. The use of electrodes with large internal surface lead to the development of models of porous electrode. structures and to the mathematical analysis of the operation of these models under certain conditions. While the chapters I to III introduce the reader to the general field offuel cells, the progress made in the understanding of the basic problems in the electrochemistry of fuel cells since the end of the second world war is reviewed in chapters IV to XVI of this monograph. In contrast, the technological aspects necessary for the development of practical units are not covered here. The open literature published as books or as papers in scientific journals has been considered up to the time of the writing of the final draft of the specific chapter, at least till the end of 1967.