The past decade has seen an explosion of information about proteoglycans (PGs). We now know that there are multiple classes of core proteins that carry cell surface heparan sulfate, others that carry cell surface chondroitin sulfate, and still others that carry heparan, chondroitin, dermatan and keratan sulfate in extracellular matrices of myriad types. We also know that there is great diversity in the structures of glycosaminoglycan (GAG) species, and that this diversity can give rise to varying degrees of specificity in interactions with proteins. We further know that the expression patterns of many PGs are highly tissue-specific, and are often dramatically regulated during development or in disease. Presumably, all this diversity of structure and expression tells us that the functions of PGs are many and varied. Yet with all the recent focus on the unique features of individual PGs, it is easy to forget that one of the factors that has long attracted researchers to the PG field is the sense-perhaps better articulated in earlier daysthat the structural commonalities among PGs underlie some shared aspects of function. We will examine here the hypothesis that the common properties of PGs render them, as a class, particularly well suited to act as catalysts in the regulation of molecular interactions. To explain this idea, it is helpful first to review some of the fundamentals of binding theory.