The rapid growth in the number of enzymes and the reactions associated with them, ensures that a wide variety of enzyme catalyzed analysis procedures are now available. The development of immobilized enzymes probes (I.E.P.) is related to advances in both immobilization technology and in the improvement of numerous sensing devices. Potentiometric, amperometric, enthalpimetric, and chemiluminescent sensors have been employed as transducers for enzyme electrodes. Most of them have been designed for organic and biological substrates for which simple analyses were not available. Nevertheless, few electrodes have been constructed for inhibitor determination. Other immobilized enzyme systems developed for analysis of enzyme inhibitors will be treated under reactor systems since they are generally used in a flow-through reactor configuration. On the other hand, if some type of electrode is used to measure the rate of an enzyme catalyzed reaction which takes place in solution, this is not a true enzyme electrode. The concept of an enzyme electrode was introduced by Hicks and Updike. The basic features of an I.E.P. are a thin enzyme layer held in close proximity to the active surface...