Glypicans are proteins with very characteristic structures that are substituted with heparan sulfate and that are linked to the cell surface via glycosylphosphatidylinositol. The modular structure of the glypicans has been highly conserved throughout evolution. Six glypicans have been identified so far in vertebrates. Mutations in Drosophila, humans and mice reveal a role for these cell surface molecules in the control of cell growth and differentiation. Their mechanism of action is not yet clear. Most likely, glypicans activate or determine the activity ranges of morphogens and growth factors such as FGFs, BMPs, Wnts, Hhs and IGFs.