Heparin and heparan sulfate glycosaminoglycans are acidic complex polysaccharides found on the cell surface and in the extracellular matrix. Recent progress has uncovered a virtual explosion of important roles of these biopolymers in fundamental biological processes. Advances in the understanding of biosynthesis and structure and the development of novel analytical methods for composition and sequence analysis have provided remarkable insights into structure/function relationships of these complex and once elusive polysaccharides.