Glycosaminoglycans (GAGs) are components of proteoglycan (PG) molecules found on the cell surface and within the extracellular matrix. PG consists of a range of core proteins that are covalently linked to one or more GAG chains. These chains are chemically defined as heparan sulfate (HS), chondroitin sulfate, dermatan sulfate, or keratan sulfate and range in molecular weight up to 100kDa. HS is generally the most abundant GAG on endothelial cell surfaces. Both the core protein and the GAG component of a PG can contribute to the molecule’s function.