Oxidation-reduction (i.e. redox) processes at the plasma membrane of any cell have been attracting more and more attention, both in basic and in applied research, since the first workshop dealing with the plasma membrane oxidoreductases was organized in Cordoba, Spain, in 1988. This evolution is evident considering the numerous cell functions performed by plasma membrane redox systems not only in healthy cells but also in cells that escaped from the normal metabolic control (e.g. cancer cells) and cells under attack by pathogens. Plasma membrane redox processes have now been demonstrated to play an essential role in growth control and defense mechanisms of these cells. The great importance of the plasma membrane redox systems originates in the fact that they are located in the membrane which is essentially the site of communication between the living cell and its environment. We may say that the plasma membrane can be considered as the "sensory part" of the cell. No chemical substance can enter the cell interior without interaction with the plasma membrane.