The genetic programs that direct formation of the treelike branching structures of two animal organs have begun to be elucidated. In both the developing Drosophila tracheal (respiratory) system and mammalian lung, a fibroblast growth factor (FGF) signaling pathway is reiteratively used to pattern successive rounds of branching. The initial pattern of signaling appears to be established by early, more global embryonic patterning systems. The FGF pathway is then modified at each stage of branching by genetic feedback controls and other signals to give distinct branching outcomes. The reiterative use of a signaling pathway by both insects and mammals suggests a general scheme for patterning branching morphogenesis.