The Drosophila midline is a critical intermediate target for navigating axons in the developing central nervous system. Several conserved families of guidance cues and their receptors, including the attractive cues Netrins and their DCC/Fra receptors and the repulsive cues Slits and their Robo receptors, regulate axon guidance at the midline. At the Drosophila midline, the Robo receptor functions to prevent ipsilateral axons from crossing the midline, and also to prevent post-crossing contralateral axons from re-crossing the midline. The Rho family GTPases—Rac, Rho and Cdc42—are key regulators of actin cytoskeletal reorganization during axon guidance. A major output of Robo downstream signaling is Slit mediated up-regulation of Rac activity. However, the link between Robo signaling and Rac activation remains unknown. In this thesis I identify CrossGAP (CrGAP), a Rac-specific GAP, as a link between the Robo receptor and Rac activation. I demonstrate that CrGAP is recruited to the plasma membrane by the Robo receptor and that Slit stimulation down-regulates the interaction between CrGAP and Robo. Our data suggest that Slit signaling down-regulates CrGAP function by regulating the subcellular localization of CrGAP, thereby up-regulating Rac activity to mediate axon repulsion.