Fig. 1. Common bioorthogonal reactions that have been applied to metabolic oligosaccharide engineering. The Staudinger ligation occurs between an azide-modified glycan and a modified triarylphosphine to yield an amide bond. The Cu(i)-catalyzed azide–alkyne cycloaddition (CuAAC) yields a stable triazole. This versatile reaction can be performed in either orientation, but azido-tags result in a lower background signal. As an alternative to CuAAC, the formation of triazoles can be promoted through ring strain in the strain-promoted azide–alkyne cycloaddition. Finally, tetrazine tags will rapidly undergo an inverse electron-demand Diels–Alder reaction with activated alkynes, like cyclopropenes.