Site-specific modification of proteins is a major challenge in modern chemical biology due to the large number of reactive functional groups typically present in polypeptides. Because of its importance in biology and medicine, the development of methods for site-specific modification of proteins is an area of intense research. Selective protein modification procedures have been useful for oriented protein immobilization, for studies of naturally occurring post-translational modifications, for creating antibody–drug conjugates, for the introduction of fluorophores and other small molecules on to proteins, for examining protein structure, folding, dynamics, and protein–protein interactions, and for the preparation of protein–polymer conjugates. One of the most important approaches for protein labeling is to incorporate bioorthogonal functionalities into proteins at specific sites via enzymatic reactions. The incorporated tags then enable reactions that are chemoselective, whose functional groups not only are inert in biological media, but also do not occur natively in proteins or other macromolecules. This review article summarizes the enzymatic strategies, which enable site-specific functionalization of proteins with a variety of different functional groups. The enzymes covered in this review include formylglycine generating enzyme, sialyltransferases, phosphopantetheinyltransferases, O-GlcNAc post-translational modification, sortagging, transglutaminase, farnesyltransferase, biotin ligase, lipoic acid ligase, and N-myristoyltransferase.