Nucleotide sugars are defined as any nucleotide in which the distal phosphoric residue of a nucleoside 5’-diphosphate is in glycosidic linkage with a monosaccharide or monosaccharide derivative. There are nine sugar nucleotides and they can be classified depending on the type of the nucleoside forming them: UDP-Glc, UDP-Gal, UDP-GlcNAc, UDP-GlcUA, UDP- Xyl, GDP-Man, GDP-Fuc and CMP-NeuNAc. In nucleotide sugar metabolism the nucleotide sugars act as donors for sugar residues in the glycosylation reactions that produce polysaccharides. They are substrates for glycosyltransferases. The nucleotide sugars are also intermediates in nucleotide sugar interconversions that produce some of the activated sugars needed for glycosylation reactions. Since most glycosylation takes place in the endoplasmic reticulum and golgi apparatus, there are a large family of nucleotide sugar transporters that allow nucleotide sugars to move from the cytoplasm, where they are produced, into the organelles where they are consumed.