Publisher Summary The purpose of this chapter is to highlight a progress in the transition metal chemistry of some enzymes that catalyze oxidative and reductive reactions. These enzymes are referred to as oxidoreductases (1,2) and transition metals are usually found in their active sites. It is essential to understand basic principles of structural organization and mechanisms of action of the key enzymes used in amperometric biosensors. These are glucose oxidase from Aspergillus niger, horseradish peroxidase (HRP), and a group of pyrroloquinoline quinine (PQQ)-dependent enzymes such as alcohol and glucose dehydrogenases. Natural substrates of HRP are aromatic amines, phenols, indoles, sulfonates, which are usually oxidized into oligomers or polymers. Mediators as electron donors act instead of H2A in the peroxidase catalysis. Their role is to rapidly reduce Compounds I and II into the resting state. Glucose oxidase from A. niger and horseradish peroxidase are the enzymes that catalyze absolutely different reactions.