Glycoproteins, that is, proteins with covalently attached sugars, are important in a myriad of biological processes and materials, including the determination of blood type, the resilience of cartilage, and virus infection. Glycoproteins may remain within the cells in which they are produced, attach to the cell surface, or be secreted. They have a diverse range of functions, including intercellular recognition, modulation of biological activity, and alteration of protein stability. The understanding of glycoproteins has lagged behind the study of other biomolecules such as DNA and RNA due to difficulties in working with glycoproteins. Biological systems produce glycoproteins as heterogeneous mixtures, and this presents practical problems in the purification of glycoproteins and the characterization of their biological effects. Currently, much research is focused on developing methods for glycoprotein synthesis that would offer access to homogeneous glycoproteins for biological and biophysical studies.