Organocatalytic protein bioconjugation has been achieved by modifying target proteins through organic ligand-assisted reactivity. Organocatalysis has been rapidly growing in the synthetic chemistry fields since the late 1990s. One of the major motivations for development of such catalytic strategies is to overcome the limitations of existing metal catalysts such as toxicity and high cost. For example, proline is a readily available, non-toxic natural amino acid that serves as a catalyst for aldol reaction and Mannich reaction. Even though a wide variety of organocatalytic transformation has been reported for small molecule substrates decades after its inception, it is interesting that protein bioconjugation driven by organocatalysis has been exclusively by acyl transfer reactions through proximityaccelerated chemistry to date. As also described in the supramolecular catalysis section, such proximity-driven chemistry functions through association and dissociation of an affinity ligand that interacts with a protein of interests. The reported organocatalytic bioconjugation processes generally proceed in two steps (Scheme 15A): ( ) association of a ligand to a target protein accelerates nucleophilic attack of ligand-tethered nucleophilic catalyst to an acyl donor reagent. ( ) Another set of nucleophilic attacks by proteins to the activated acyl donor occurs, followed by dissociation of the ligand-catalyst. The following paragraphs focus on evolution of acyl transfer-based organocatalytic bioconjugation methods and brief demonstration of their utilities and applications.