The basic structure as well as the physical existence of the MOS field-effect transistor is without doubt of great importance for the development of a whole series of sensors for the measurement of physical and chemical environmental parameters. The equation for the MOSFET drain current already shows a number of parameters that can be directly influences by an external quantity, but small technological variations of the original MOSFET configuration also give rise to a large number of sensing properties. All devices have in common that a surface charge is measured in a silicon chip, depending on an electric field in the adjacent insulator. FET-based sensors such as the GASFET, OGFET, ADFET, SAFET, CFT, PRESSFET, ISFET, CHEMFET, REFET, ENFET, IMFET, BIOFET, etc. developed up to the present or those to be developed in the near future will be discussed in relation to the considerations mentioned above.