During embryonic lung development, interacting signalling molecules made by the developing lung's outer mesothelium, mesenchyme and bronchial epithelium regulate lung growth and morphogenesis. Fibroblast growth factor 9 (FGF9),which is expressed in the mesothelium and epithelium during early lung development, and sonic hedgehog (SHH), which is expressed only in the epithelium, are required for both processes. On p. 1507, David Ornitz and colleagues use Fgf9 loss-of-function and inducible gain-of-function mouse models to show that FGF9 and SHH signalling coordinate lung growth and development by regulating two distinct mesenchymal domains. They report that mesothelium-derived FGF9 signals stimulate proliferation in sub-mesothelial mesenchyme and that epithelium-derived FGF9 signals regulate SHH signalling in a distinct sub-epithelial mesenchyme domain; SHH signalling subsequently regulates cell proliferation and survival, and the expression of mesenchyme-to-epithelial signals. The researchers' report of distinct mesenchymal domains in the developing lung, which respond to different combinations of signalling molecules, is a novel finding that is of relevance to the study of other developing organs.