Glycosphingolipids (GSLs), cell type-specific markers which change dramatically during ontogenesis and oncogenesis, have been implicated as playing major roles in cellular interactions and control of cell proliferation in multicellular organisms. These functional roles have been partially clarified through two types of studies: (i) Studies of cell recognition mediated by (a) GSL-GSL interaction, (b) GSL-lectin interaction, and (c) GSL-dependent modulation of integrin receptor function. (ii) Studies on control of transmembrane signaling by GSLs and/or sphingosine (Sph) derivatives, with emphasis on effects of these compounds on: (a) signaling pathways initiated by tyrosine kinase-linked receptors; (b) signaling systems mediated by protein kinase C, MAP kinase, other kinases, or cytosolic Ca2+ concentration, leading to changes in cellular phenotypes such as motility, proliferation, differentiation, and apoptosis.