Thomas M. Jessel, Howard Hughes Medical Institute Investigator, explores the human brain, the sophisticated product of 500 million years of vertebrate evolution, assembled during just nine months of embryonic development. The functions encoded by its trillion nerve cells direct all human behavior. Yet the brain is a biological organ made from the same building blocks as skin, liver and lung. How does the brain acquire its remarkable computational power? Answers lie in the details of its construction — the cellular and molecular mechanisms that drive the formation of thousands of neural circuits, each wired for a specific behavior.