Intense research over the past four years has led to the discovery and characterization of a novel signalling network, known as the Salvador–Warts–Hippo (SWH) pathway, involved in tissue growth control in Drosophila melanogaster. At present, eleven proteins have been implicated as members of this pathway, and several downstream effector genes have been characterized. The importance of this pathway is emphasized by its evolutionary conservation, and by increasing evidence that its deregulation occurs in human tumours. Here, we review the main findings from Drosophila and the implications that these have for tumorigenesis in mammals.