The JAK/STAT signal transduction pathway plays a critical role in mammalian cells, particularly in hematopoiesis and immune responses. Several components of this pathway have been identified and characterized in Drosophila. Mutational analyses of these components have revealed a number of interesting developmental roles, and provide a mechanism to identify other interacting molecules and pathways. Hence, the JAK/STAT pathway in Drosophila serves as an attractive model for in vivo functional analyses of JAK/STAT signaling.