This review will summarize our current state of knowledge of the structure, biochemical properties and functions of syndecans, a family of transmembrane heparan sulphate proteoglycans. Syndecans bind a variety of extracellular ligands via their covalently attached heparan sulphate chains. Syndecans have been proposed to play a role in a variety of cellular functions, including cell proliferation and cell-matrix and cell-cell adhesion. Syndecan expression is highly regulated and is cell-type- and developmental-stage-specific. The main function of syndecans appears to be to modulate the ligand-dependent activation of primary signalling receptors at the cell surface. Principal functions of the syndecan core proteins are to target the heparan sulphate chains to the appropriate plasma-membrane compartment and to interact with components of the actin-based cytoskeleton. Several functions of the syndecans, including syndecan oligomerization and actin cytoskeleton association, have been localized to specific structural domains of syndecan core proteins.