Glycosaminoglycans are negatively charged polysaccharides composed of repeating disaccharide units. Sulfate groups may be attached to the carbohydrate backbone and hence contribute to the polyanionic properties of the molecule. In the tissues the glycosaminoglycans (with the exception of hyaluronic acid) occur as proteoglycans that are composed of one or more polysaccharide chains attached to a core protein. During the last decade the presence of glycosaminoglycans at the surface of cells has been demonstrated in several systems. Owing to this location, proteoglycans have been postulated to play a role in cell-cell and cell-substrate interactions. Although several studies support this hypothesis, direct evidence for an involvement of cell-associated proteoglycans in cell-contact phenomena has not yet been presented. In a few cases it has been demonstrated that cell-associated glycosaminoglycans may act as 'receptors' for circulating components. Future studies will undoubtedly reveal the physiological functions that cell-associated glycosaminoglycans perform. The number of tissues and types of cultured cells from which proteoglycans have been isolated has increased dramatically. In the years to come these proteoglycans will be characterized and their structures compared. The structures of the core proteins will receive major attention and studies of the corresponding genes will provide us with information about the relations between different proteoglycans. The purpose of this chapter is to provide an overview of the structure, biosynthesis, and classification of proteoglycans in general and to examine in somewhat more detail the structure, interactions, and possible functions of glycosaminoglycans located at the surface of cells.