Intercellular interactions have a major role in regulating a wide variety of developmental processes. It is now evident that the molecules that mediate these interactions are members of a relatively small number of families of secreted" growth factors" or" ligands" that transmit signals by activating receptors on the cell surface of neighboring cells. The fibroblast growth factor (FGF) family of heparin-binding proteins is one such group of signaling molecules. Here, we briefly summarize current knowledge about the FGF ligands and their receptors and then focus attention on one member of this gene family, Fgf8, which is thought to have key roles in a variety of developmental processes, including limb and brain development. We then describe the genetic approach we have taken to explore Fgf8 gene function in the mouse.