Heparan sulphate (HS) is an abundant component of cell surfaces and the extracellular matrix. It binds to a wide variety of peptide growth factors, morphogens, chemokines and extracellular matrix proteins (e.g. fibronectin) and many of these interactions are essential for these effector proteins to transduce signals across the plasma membrane. The unique molecular design and flexibility of HS are essential for its ability to exert control over the cellular response to proteinaceous ligands. The clustering of sulphated sugar residues in a series of complex domains with variable sulphation patterns generates considerable diversity in the molecular fine structure of HS. This diversity reflects a high degree of selectivity in protein recognition and in the assembly of functional multiprotein complexes on the HS polymer chain.