Branching morphogenesis occurs during the development of many vertebrate organs. In developing mouse salivary glands, for example, deep clefts that form on the surface of primary buds subdivide them into secondary buds, which then grow out. This process is thought to be driven by cell migration and is repeated multiple times to produce a multilobular, branched structure - the buds then develop into secretory units connected by hollow ducts. Melinda Larsen, Cindy Wei and Kenneth Yamada now reveal that branching morphogenesis involves both cell movement and directional assembly of the matrix protein fibronectin (see p. 3376). The authors report that during branching morphogenesis individual epithelial cells in the salivary gland bud move rapidly but randomly. These movements alone cannot explain the highly ordered process of branching morphogenesis. Instead, examination of fibronectin dynamics indicates that this matrix polymer, which is essential for branching morphogenesis, assembles a wedge at the base of developing clefts that moves into the gland. In this way, fibronectin promotes the separation of the actively jostling epithelial cells to facilitate cleft formation and branching morphogenesis.