Copper(I) is able to catalyze Huisgen 1,3-dipolar cycloaddition in a "click" fashion. This copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction presents excellent chemoselectivity and occurs over a wide-range of reaction conditions. It shows tolerance to variation in both pH and solvent polarity, thereby facilitating the ligation of peptides and proteins to produce peptidomimetics and synthetic proteins. In addition, the only product formed is a 1,4-disubstituted-1,2,3-triazole moiety, in many aspects resembling the natural peptide bond, including hydrogen-bonding capability, planarity, distance between the 1 and 4 substituents, and conformational restriction of the peptide backbone; thus the triazole-backbone-modified peptide, in which a triazole replaces the amide bond, may be anticipated to present a secondary structure similar to that of its natural counterpart. This Focus Review describes the scope and applications of copper(I)-catalyzed alkyne–azide cycloaddition in synthetic peptide/protein chemistry.