Despite their diversity, and perhaps because of it (i.e. because that diversity implies more information content) sialic acid glycans of host cells are key molecular recognition features not only for entry of viruses such as influenza, but also in embryonic development, neurodevelopment, reprogramming, and oncogenesis. Correctly speaking, even sialic acid itself is diverse. It is a generic term for a family of derivatives of the nine-carbon sugar neuraminic acid. The sialic acid family includes some 43 derivatives of neuraminic acid, but these acids rarely appear free in nature. Members include N-acetylneuraminic acid, 2-keto-3-deoxy-d-glycero-d-galacto-nonulosonic acid, 5,7-diamino-3,5,7,9-tetra-deoxy-d-glycero-d-galacto- nonulosonic acid, and 5,7-diamino-3,5,7,9-tetra-deoxy-l-glycero-l-manno-nonulosonic acid. If the term “sialic acid” is used unqualified, it usually refers to the representative member of this group, N-acetylneuraminic acid. The variability of glycans is not random but reflects their modes of synthesis. In eukaryotes generally, a typical N-linked glycan has an initial core that consists of 14 residues (3 glucose, 9 mannose, and 2 N-acetylglucosamine). This preassembled glycan is usually transferred by a glycosyltransferase oligosaccharyltransferase to a nascent peptide chain within the reticular lumen. This initial core 14-sugar unit is assembled in the cytoplasm and endoplasmic reticulum and other sugars may be added later. In contrast, O-linked glycans are assembled one sugar at a time at the outset on proteins in the Golgi apparatus.