Publisher Summary This chapter describes the structure of the active surface of cholinesterases and presents the mechanism of their catalytic action in ester hydrolysis including enzyme sources, various methods, catalytic effects of cholinesterases, mechanism of hydrolysis by cholinesterases, structure of the esteratic site, and the anionic sites in the active centers of cholinesterases. Enzymatic ester hydrolysis is a common and widespread biochemical reaction. The extensive studies on substrate and inhibitor specificity, on kinetics of hydrolysis and on the influence of pH variations on the reactions catalyzed by cholinesterases have given very instructive information on the structure of the active surface and the mechanism of enzymatic hydrolysis. It illustrates the combination of the cationic substrate acetylcholine with the active surface of true ChE and demonstrates the double anionic site, the imidazol ring, and the phenolic hydroxyl as involved in the binding of the substrate by electrostatic, covalent, and hydrogen bonds. The hydroxyl group of serine is shown in close proximity, but in a nonbonding position. In spite of the enormous effort devoted to these studies, the picture remains entirely hypothetical, because the high molecular weight of these enzymes and the lack of a specific prosthetic group make isolation of the components of the active surface extremely difficult and permit only an indirect approach to this problem.