The four syndecans identified in mammals are membrane proteoglycans that play major roles in regulating cell behavior, cell signaling, and cell‐matrix interactions. The membrane forms of these syndecans function as receptors and co‐receptors. Their ectodomains, which are proteolytically released in the extracellular matrix by shedding, also regulate various biological processes. Apart from the cytoplasmic domain of syndecan‐4, the 3D structures of syndecans are poorly characterized, which hinders our understanding of the molecular mechanisms underlying syndecan functions that are mediated by numerous interactions. This mini‐review summarizes the structural data that are available for syndecans and provides a comprehensive syndecan interactome, which comprises three hundred and fifty‐one partners, including those identified by the high‐throughput method affinity purification‐mass spectrometry. It also gives a perspective on future studies of syndecan structures and interactions, which are required to further elucidate the molecular recognition processes that mediate the biological roles of the membrane and shed forms of syndecans.