Basement membranes lie at the epithelial-mesenchymal interface of most tissues. These thin layers of highly specialized extracellular matrix vary in composition in different tissues and also over the course of tissue morphogenesis. Heparan sulfate proteoglycans, which were originally identified in basement membranes, interact with extracellular matrix proteins, growth factors and cell receptors, and influence cellular signaling. Members of this family in the human placenta and decidua that act principally in linking to collagen IV and laminin networks include perlecan, agrin, and collagen XVIII, each of which have characteristic locations. Perlecan is widely expressed in trophoblasts, the villous and endothelial basement membranes, villous stroma, and decidua, whereas collagen XVIII is not expressed in trophoblasts. Agrin expression is quite limited, occurring only in the decidua and villous stroma. Pathological conditions may alter the expression and structure of the covalently attached glycosaminoglycan chains of these molecules in the placenta. Such changes may result in remodeling of the basement membrane during placental development with consequent adverse effects, as seen for example in gestational diabetes and other diseases or experimental models.