Drosophila Slit and its vertebrate orthologues Slit1–Slit3 are secreted glycoproteins that play important roles in the development of the nervous system and other organs. Human Slits are also involved in a number of pathological situations, such as cancer and inflammation. Slits exert their effects by activating receptors of the Robo (Roundabout) family, which resemble cell adhesion molecules in their ectodomains and have large, mainly unstructured cytosolic domains. HS (heparan sulfate) is required for Slit–Robo signalling. The hallmark of Slit proteins is a tandem of four LRR (leucine-rich repeat) domains, which mediate binding to the IG (immunoglobulin-like) domains of Robos. A major question is how Slit binding is translated into the recruitment of effector molecules to the cytosolic domain of Robo. Detailed structure–function studies have shown that the second LRR domain of Slit (D2) binds to the first two IG domains of Robo, and that HS serves to stabilize the Slit–Robo interaction and is required for biological activity of Slit D2. Very recently, the crystal structure of a minimal Slit–Robo complex revealed that the IG1 domain of Robo is bound by the concave face of Slit D2, confirming earlier mutagenesis data. To define the mechanism of Robo transmembrane signalling, these structural insights will have to be complemented by new cell biology and microscopy approaches.