Metal-catalyzed protein modification has been enabled through a series of chemical strategies such as bioorthogonal chemistry and design/utilization of protein-compatible transition-metal complexes. A number of metal-mediated protein modification were achieved by use of noncanonical amino acids. For instance, copper-catalyzed azide-alkyne cycloaddition-a quintessential "click" reactionhas been widely used by introducing an azide or alkyne handle onto proteins through methods such as chemical modification and genetic/metabolic incorporation. Similarly, instillation of arylboronic acid or aryliodide allows for Suzuki-Miyaura coupling with a palladium catalyst. Olefin metathesis with a Grubbs-type ruthenium complex is also possible by introduction of noncanonical alkene groups with a thioether moiety. A large portion of metal-mediated modification of canonical amino acid residues, on the other hand, is often a non-catalytic system. Even for catalytic reactions of natural proteins, an excess amount of metal catalysts is necessary likely because of interaction of proteins with metal salts and because of challenges of catalysis in aqueous media. Indeed, there are many catalytic systems that uses organic solvents that are typically not compatible with protein substrates (but peptides). Examples described below tackled the challenges of protein modification in aqueous solutions by modulation of reactivity of a metal catalyst such as use of a coordinatively saturated complex to avoid undesired interactions with proteins. As there have been multiple review articles about metal-based protein bioconjugation in the past, this section will be focused on catalytic aspects of the processes. The metal catalysis leveraged by their Lewis acidic nature is discussed in the Acid catalysis section.