(A) Most mammalian epithelial tubes have a simple organization. However, during active morphogenesis both the mammary [14] and salivary [26] epithelia are multilayered or stratified. By contrast lung appears to retain a simple organization during branching morphogenesis [40]. (B) During branching morphogenesis in each of the epithelial organs there is a rich diversity of ECM proteins and stromal cell types, including diverse leukocytes and fibroblasts. Functional evidence has been provided for the importance of macrophages during mammary development [22,26], but it remains a relatively open question which stromal cell types are required for, or assist in, the normal development of the epithelial organs. ECM = extracellular matrix.