Slits are multifunctional guidance cues, capable of triggering neurite repulsion, extension, or branching, depending on cell type and developmental context. While the Robo family of Slit receptors is a well-established mediator of axon repulsion, a role for Robos in Slit-mediated neurite growth and branching is not well defined, and the signaling molecules that link Robo to the cytoskeletal changes that drive neurite outgrowth are not well characterized in vertebrates. We show that Slit stimulates cortical dendrite branching, and we report that Slit also triggers a robust increase in the length of cortical axons in vitro. Moreover, neurons derived from Robo1; Robo2 deficient mice do not display an increase in neurite length, indicating that endogenous Robos mediate Slit's growth-promoting effects on both axons and dendrites. We also demonstrate that the SH2/SH3 adaptor proteins Nck1 and Nck2 bind to Robo via an atypical SH3-mediated mechanism. Furthermore, we show that only Nck2 is required for the Slit-induced changes in cortical neuron morphology in vitro. These findings indicate a specific role for Nck2 in linking Robo activation to the cytoskeleton rearrangements that shape cortical neuron morphology.