This review explains how growing nerve fibers find their way in the developing brain and spinal cord using special signalling molecules called Slit and Robo. These molecules work together like a navigation system, with Slit acting as a ‘stop’ signal secreted from midline structures and Robo receptors on growing axons receiving these signals. The same system is used by flies, worms, and humans, showing that this guidance mechanism is an ancient and essential part of nervous system development.