9-Azidosialic acid (9-azido-Neu5Ac) is an azide-bearing sialic acid analog used in metabolic glycoengineering and chemical reporter strategies. When supplied to cells (typically as a membrane-permeable peracetylated precursor), it is incorporated into the sialic acid biosynthetic pathway and displayed on cell-surface sialoglycans, where the azide handle can be detected or tagged via bioorthogonal click chemistry — copper-catalyzed azide-alkyne cycloaddition (CuAAC) with alkynes or copper-free strain-promoted cycloaddition (SPAAC) with DBCO and related strained alkynes — enabling sialoglycan imaging, enrichment, and glycoproteomic analysis.