(A) Examples of using N-azidoacetylmannosamine (ManNAz) or per-O-acetylated ManNAz (Ac4ManNAz) as metabolic precursors of N-azidoacetylneuraminic acid (Neu5Az) are shown. The reaction of a metabolically engineered azido-containing sialic acid with a terminal alkyne-containing compound (i) by copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) in the presence of a tris(triazolylmethyl)amine-based ligand forms a triazole-linked product. ManNAz is taken up by cells, converted to activated Neu5Az, which is transferred to cell surface glycoproteins as an azido-containing derivative of the most common sialic acid, Neu5Ac. Per-O-acetylation of monosaccharides enhances their uptake by cells. O-Acetyl groups are removed by cytoplasmic esterases, and the azido-containing monosaccharides go through metabolic processes to present on the glycan components of glycoconjugates. CuAAC leads to visualizing glycan changes in live cells, tissues, or organisms and can be applied for drug delivery, diagnosis, and functional studies of the important roles of glycans. (B) Additional per-O-acetylated monosaccharide precursors that have been explored, such as per-O-acetylated N-azidoacetylgalactosamine (Ac4GalNAz), N-azidoacetylglucosamine (Ac4GlcNAz), and 6-azido-L-fucose (Ac4Fuc6N3). (C) Additional representative bioorthogonal reaction counterparts of azides, including novel triarylphosphine (ii), difluorinated cyclooctyne (iii), and biarylazacyclooctynone (BARAC) (iv) probes. Colors and shapes of monosaccharides follow the Symbol Nomenclature for Glycans (31).