Abstract: The Huisgen azide−alkyne 1,3-dipolar cycloaddition of azides and alkynes has had a profound impact on chemical biology. The diazo group shares this reactivity with the azido group while conferring additional versatility. A diazo compound can be accessed readily from its parent azide by simple deimidogenation, that is, loss of “NH”, using a phosphine reagent. Cycloaddition with a diazo compound can be tuned to be much faster or much slower than that with its analogous azide. Like azides, diazo compounds endure metabolism. Moreover, the chemoselectivity of diazo and alkynyl groups enables dual-labeling experiments that are not possible with azido and alkynyl groups. Perhaps most importantly, diazo compounds offer entirely new modes of reactivity, including the O-alkylation of carboxylic acids in water to form esters. As in prodrugs, such esterification enables a means to modify natural proteins covalently but bioreversibly with desirable pendants, including ones that enable uptake directly into the cytosol of mammalian cells.