Fibroblast growth factors (FGFs) comprise a large family of developmental and physiological signaling molecules. All FGFs have a high affinity for the glycosaminoglycan heparin and for cell surface heparan sulfate proteoglycans. A large body of biochemical and cellular evidence points to a direct role for heparin/heparan sulfate in the formation of an active FGF/FGF receptor signaling complex. However, until recently there has been no direct demonstration that heparan is required for the biological activity of FGF in a developmental system in vivo. A recent paper by Lin et al.(1) has broken through this barrier to demonstrate that heparan sulfate is essential for FGF function during Drosophila development. The establishment of a role for heparan sulfate in FGFR activation in vivo suggests that tissue-specific differences in the structure of heparan may modulate the activity of FGF. BioEssays 22: 108–112, 2000.© 2000 John Wiley & Sons, Inc.