A review. Proteoglycans (PGs) and glycosaminoglycans (GAGs) have been shown to be key mols. in physiol. and consequently their modulations have been observed in various pathologies. Here, multiple facets of their structures and functions, which have raised new opportunities in drug development, are described. PGs and GAGs are mainly classified according their localization at the cell membrane, in the extracellular matrix, being intracellular or circulating in the blood. Structurally, PGs are macromols. which consist of a core protein and associated GAG chains. The different GAG chains of PGs, heparan sulfate/heparin, dermatan/chondroitin sulfate, and keratan sulfate are synthesized as long polymers of repeating disaccharide units. They are strongly implicated in the different stages of development, cancer, or inflammation. For instance, the repertoire of PGs and GAGs (natures and structures) has been shown to change continuously and dynamically during the progression of processes such as the inflammatory response or the development of tumors. These changes include the modulation of the activity of GAG-binding cytokines, growth factors, proteases, and protease inhibitors. All of the interactions of regulatory proteins (e.g., growth factors, chemokines, enzymes) with GAGs provide much of the focus for GAG-based therapeutic targets. GAG mimetics should therefore have clin. applications as modulators of cytokine, growth factor, or enzyme function in various diseases and pathologies.