Interest in chemical sensors, as in other sensors, is rapidly growing due to increasing demand from a broad application field including industrial production and environment monitoring. Over the past years, the world market has shown average annual increases of around 10%. New technologies have also improved sensor versatility. Association with small computers transforms otherwise unviable devices into “smart” sensors. Integrated ChemFETs are very attractive, offering adequate miniaturisation for biological and medical use and a potential for mass production. The ISFET, first introduced by Bergveld 1, is simply derived from a MOSFET, with the metallic gate replaced by an ion-sensitive membrane. In the hydrogen GasFET 2 the gate is made of palladium. The CO-GasFET designed by Krey et al 3 illustrates the pragmatic approach which prevails in the field. Pure ionic and mixed conductors can assume various functions in chemical sensors. Solid electrolytes are the most frequently used as sensitive membranes in potentiometric cells. For this application, the technology and the problems to be solved markedly depend on the nature of the analyzed-medium electrical conductivity (either electronic, ionic or insulating). A detailed investigation shows that the electronic conductivity of the sensitive membrane is far less detrimental in ion analysis than, for instance, in gas analysis. Even mixed conductors can be used, although redox reactions may then interfere. Ion (and electron) Fermi level anchoring is frequently a major problem in chemical sensors. Conventional internal reference electrodes are not always compatible with the semiconductor components of the ChemFET and other anchoring techniques must be developed. An ionic bridge made of a mixed alkali ion/Ag + conductor, for instance, can insure a transition from an alkali sensitive membrane to a silver internal electrode compatible with the FET. The utilisation of a mixed conductor is another solution with additional advantages. With such a sensitive membrane only one coating is required to form an ISFET. Furthermore, the charge of the membrane can be varied for optimum adaptation to the various solutions to be analyzed. Another situation where Fermi level anchoring is crucial is found in advanced gas sensors 4 involving solid electrolytes conducting by alio-ion (e.g. F − ion conducting membranes used in O 2 sensors). Doping the catalytic electrode by the conducting ions (F − in the above example) improves stability of the electrode potential. The analysis of possible interference reactions demonstrates that 3-D fast ion conductors are the best candidates for ion sensitive membranes. Tests carried out with NASICON as a Na + sensitive membrane are reported. Interference with pH is indeed significantly smaller than with commercial MAS glasses.