Syndecans are type 1 transmembrane proteoglycans that constitute the major physiological form of heparan sulfate (HS) on the cell surface. The four vertebrate syndecans, Syndecan-1 through -4, have similar short cytoplasmic domains and extracellular portions that diverge, except for HS attachment sites. Structurally diverse side chains add considerably to the size of the core proteins and serve as binding sites for growth factors, cytokines, and extracellular matrix proteins. Syndecans are present as homodimers or multimers, and are often expressed in developmental and cell type-specific patterns. They have many biological roles including regulating cell growth, differentiation, and adhesion.