Enzymes catalyze two types of modifications of proteins, that of hydrolytic and of nonhydrolytic modifications. These modifications, occurring in vivo during or after translation of the protein, are highly specific and may affect markedly the biological, chemical, and physical properties of the protein. Specific proteolysis can be a control process, allowing the biological activity of a number of proteins to be expressed. It also is an intracellular control process since proteins are continuously undergoing degradation at rates that are highly specific for each protein. Many highly specific proteases must occur in rather small amounts in vivo in order to account for the multiplicity of physiological roles in which they are involved. This chapter examines primarily some of the hydrolytic reactions that occur in vivo in the hope that some of the knowledge can be adapted to the hydrolytic modification of food proteins.T^nzyme-catalyzed modifications of proteins deserve much more work -*-/ and therefore must be considered as an important field for further scientific investigations. In spite of the prolific research carried out in this area during the past three decades, a number of enzymatic modifi cations not only are poorly investigated or understood but attempts to apply them to food protein systems are nonexistent. The fundamental aspects of enzymatic modification of proteins are of interest since potential applications for nutritional and functional improvements of food proteins appear to be numerous and promising. Enzymatic and chemical modifi-0065-2393/82/0198-0057$07.75/0