Our understanding of organ growth control during development has recently been given a boost by the discovery of the Hippo signalling pathway in Drosophila. This phosphorylation cascade is required for imaginal disc, the organ precursors, to stop growing at the end of larval life; indeed, mutations in the genes encoding the kinases of this pathway, or in their interactors, lead to organ overgrowth. The Hippo pathway acts in repressing the transcription of target genes promoting proliferation and survival. This pathway is thought to integrate many upstream signals, although this is only partially understood. Altogether, integration of these inputs enables a tight control of cell number within organs and hence of organ size. As this pathway is conserved in mammals, it offers new research opportunities to better understand and fight cancer.