The JAK-STAT pathway is a key regulator of tissue size in Drosophila melanogaster. Here we provide an overview of its roles in processes that regulate the size of Drosophila imaginal discs, epithelia of diploid cells that proliferate and acquire specific fates in the larvae and that become functional in the adult. Drosophila has a single JAK and a single STAT gene, which has facilitated genetic dissection of this pathway. Moreover, the sophisticated genetic tools available in flies for clonal growth assays have made Drosophila an ideal organism in which to dissect the multiple roles of the JAK-STAT pathway in growth control. Studies in flies have revealed JAK-STAT pathway activity as a central node for diverse signals that control proliferation and mass accumulation. In addition, recent work has establish a new role for the pathway in cell competition, a process thought to be akin to the early stages of transformation in which more robust cells kill and take the place of less robust ones.