Proteoglycans are produced by most eukaryotic cells and are versatile components of pericellular and extracellular matrices. They belong to many different protein families. Their functions vary from the physical effects of the proteoglycan aggrecan, which binds with link protein to hyaluronan to form multimolecular aggregates in cartilage; to the intercalated membrane protein CD44 that has a proteoglycan form and is a receptor and a cell-binding site for hyaluronan; to heparan sulfate proteoglycans of the syndecan and other families that provide matrix binding sites and cell-surface receptors for growth factors such as fibroblast growth factor (FGF). One feature that recurs in proteoglycan biology is that their structure is open to extensive modulation during cellular expression. Examples of protein changes are known, but a major source of structural variation is in the glycosaminoglycan chains. The number of chains and their length can vary, as well as their pattern of sulfation. This may result in the switching of different chain types with different properties, e.g., chondroitin sulfate and heparan sulfate, and it may also result in the selective expression of sulfated chain sequences that have specific functions. The control of glycosaminoglycan structure is not well understood, but it does appear to be used to change the properties of proteoglycans to suit different biological needs. Proteoglycan forms of proteins are thus important modifiers of the organization of the pericellular and extracellular matrices and modulators of the processes that occur there.