The purpose of this chapter is to present a brief overview of the potentiometric sensors available for applications in high temperature liquids. The topics addressed include: the calculation rules of the e.m.f. of the electrochemical chain, with special emphasis on sources of errors, a discussion on the reference electrodes of the cells according to the temperature and the types of analyzed solutions. Finally, the state of the art of the potentiometric sensors available for monitoring of oxygen, hydrogen and carbon in molten metals and for studying oxoacidic properties of molten salts is presented.