The syndecan family of cell-surface heparan sulfate proteoglycans participate in multiple cell behaviors ranging from growth factor signaling to cell adhesion. Participation in these activities is dependent on specific binding interactions of their heparan sulfate chains and molecular interactions of their core proteins with cytoskeletal and signaling molecules. The highly conserved features of the core proteins have long suggested important functions, which are only now beginning to be understood. Recent advances point to important roles for the extracellular, transmembrane and cytoplasmic domains of the syndecan core proteins in the assembly of these proteoglycans into an intracellular cytoskeletal and signaling apparatus. The proteins display interactions that may be common among the different family members, as well as interactions that provide signaling capabilities that are specific to individual members.