A classification of the possible applications is given first. For certain cases, the emf of a concentration cell is the dominant quantity as in some sensors. For other cases, the protonic current is of interest. In some applications the chemical reactions at the cathode and anode are of importance, while in further areas only the fact of relevance is that protons are transported to the cathode. The domain of sensors of hydrogen, steam and hydrogen containing organics is covered. The first commercial application of oxidic high-temperature proton conductors, the quantitative detection of hydrogen in molten metals, belongs to that group. In the fuel cell area, prototypical high-temperature fuel cells based on proton motion are discussed. Important here is the distinction between thick, medium thickness and thin film electrolytes. As in the case of 8YSZ, high current densities can only be obtained with the latter. Proton conductors also allow the pumping of hydrogen and steam. Novel vacuum leaks for the introduction of hydrogen into vacua are presented as a case in point. A combination of proton and oxygen ion conductors permits the in situ generation of steam, whereas the reverse, steam electrolysis, allows the production of high purity hydrogen. (C) 2003 Elsevier B.V. All rights reserved.