Glycosylphosphatidylinositol (GPI) moieties consist of a hydrophobic core (two to three fatty acids attached to phosphatidylinositol) continued by a chain of five hexoses (the glycolinker) that is covalently linked via phosphoethanolamine to the C-terminal amino acid of a long list of mammalian proteins. This glycolipid moiety imparts characteristic membrane properties (diffusion, lateral mobility and clustering) to proteins and the glycolinker component constitutes a molecular motif recognized by lectins, cytokines and bacterial toxins. Due to the saturated acyl chains of its core lipids, GPIs preferentially insert in sphingolipid-rich domains of the plasma membrane outer leaflet, and the capacity of GPIanchored proteins to participate in transmembrane signalling depends on this property. GPI-anchored proteins can be made to associate with membranes and thus offer a therapeutic potential in tumor vaccine preparation and complement protection. Research on the role of GPI-anchored proteins in membrane structure and function has fruitfully contributed to the elaboration of today ’ s prevailing concepts of membrane organization.