(Molecular biology) The addition of carbohydrate groups such as N-acetylglucosamine, mannose and glucose to proteins. Glycosylation occurs in two forms: N-linked (in which a preformed oligosaccharide unit is attached to a nitrogen atom in a asparagine residue) and O-linked (in which the sugars are attached to the hydroxyl group of a serine or threonine residue).

N-linked glycosylation plays an important role in the transport of membrane proteins from the endoplasmic reticulum, where they are synthesised, to their target organelles. Different combinations of sugars act as "postcodes," directing the proteins to the areas of the cell where they perform their biological function. Defects in glycosylation underlie a number of diseases such as Jaeken disease, congenital dyserythropoietic anemia type II and various forms of muscular dystrophy.