Marc Tessier-Lavigne, PhD, president of Stanford University in Stanford, Calif., made groundbreaking contributions to understanding the molecular underpinnings of axon guidance. Working with the mammalian spinal cord, Tessier-Lavigne, identified multiple guidance systems and showed how they work together to effect accurate guidance. He biochemically purified Netrins and Slits, two major evolutionarily conserved families of guidance cues, which were also independently identified in invertebrate systems. He dissected the guidance of axons to and beyond the spinal cord midline, showing that axons are attracted to the midline by Netrins; that Netrins collaborate with a distinct midline attractant as well as a repellent that helps prevent the axons from straying from their path; and that leaving the midline involves the action of Slits and Semaphorins. Other major discoveries included the identification of vertebrate receptors for Netrins, Slits, Semaphorins and other cues, and of additional chemoattractants for diverse axonal populations. Tessier-Lavigne’s use of evolutionary insights combined with penetrating mechanistic and molecular analyses dramatically transformed scientific thinking about neural development.