A glycoprotein is a macromolecule composed of a protein and a carbohydrate (an oligosaccharide). The addition of sugar chains can happen either at asparagine, and is termed N-glycosylation?, or at hydroxylysine, hydroxyproline, serine, or threonine, and is termed O-glycosylation?. Possible carbohydrates include glucose, glucosamine, galactose, galactosamine, mannose, fucose, and sialic acid.
The sugar group can assist in protein folding or improve its stability. Glycoproteins are often used in proteins that are at least in part located in extracellular space (that is, outside the cell). Glycoproteins are important for immune cell recognition, especially in mammals. 
Examples of glycoproteins in the immune system are:
- Molecules such as antibodies (immunoglobulins), which interact directly with antigens
- Molecules of the major histocompatibility complex (or MHC), which are expressed on the surface of cells and interact with T-cells as part of the adaptive immune response.
Other examples of glycoproteins include:
- Components of the zona pellucida, which surrounds the oocyte, and is important for sperm-egg interaction.
Soluble glycoproteins often show a high viscosity, for example, in egg white and blood plasma.
Hormones that are glycoproteins include:
- Follicle stimulating hormone
- Luteinizing hormone
- Thyroid stimulating hormone
- human chorionic gonadotropin
Remarkably little is known about the evolution of glycosylation. There are clearly shared and unique features of glycosylation in different kingdoms and taxa, and among animals, an increasing complexity is often seen in higher forms. Intra- and interspecies variations in glycosylation are also relatively common. It has been suggested that the more specific biological roles of oligosaccharides are often mediated by unusual glycan structures, unusual presentations of common structures, or further modifications of the saccharides themselves. Such structures likely result from the unique expression patterns of the relevant glycosyltransferases. However, such glycans are also more likely to be targets for recognition by pathogenic toxins and microorganisms.()