The JAK/STAT pathway was originally identified in mammals. Studies of this 
pathway in the mouse have revealed that JAK/STAT signaling plays a central role 
during hematopoeisis and other developmental processes. The role of JAK/STAT 
signaling in blood appears to be conserved throughout evolution, as it is also 
required during fly hematopoeisis. Studies in Dictyostelium, Drosophila, and 
zebrafish have shown that the JAK/STAT pathway is also required in an unusually 
broad set of developmental decisions, including cell proliferation, cell fate 
determination, cell migration, planar polarity, convergent extension, and 
immunity. There is increasing evidence that the versatility of this pathway 
relies on its cooperation with other signal transduction pathways. In this 
review, we discuss the components of the JAK/STAT pathway in model organisms and 
what is known about its requirement in cellular and developmental processes. In 
particular, we emphasize recent insights into the role that this pathway plays 
in the control of cell movement.