Chemical sensors are increasingly used in numerous applications, such as analysis setups, chemical processes, food industry, environmental control, and biomedical applications. The fundamental principles of potentiometric sensors based on solid electrolytes have been reviewed in several papers. In ionic sensors, such systems are called ionic bridge or internal reference. As the second electrode, an external reference is used; for instance, a calomel electrode with a liquid junction. The choice of the working temperature depends on the conductivity of the solid state chemistry and on the electrode kinetics. Amperometric sensors are based on electrochemical reactions which are governed by the diffusion of the electroactive species through a barrier. Potentiometric sensors are very well suited to perform measurements in real time. For servo control, low response times avoid oscillation phenomena. Most solid state electrochemical sensors are interesting from a theoretical point of view because, generally, the responses obey thermodynamic laws.