The human brain is 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—from the simple movements of everyday life to the daring and inspirational thoughts that sometimes emerge. 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. This lecture delves into the developmental programs that control brain wiring to understand the cues that trigger neurons to take the correct shape and connect with appropriate partners. As the genetic blueprint for brain wiring unfolds, early experience validates neural networks by frequent use, sculpting the final pattern of neural connections and thus enabling and constraining our behavior. This lecture also explores how understanding neural circuit assembly suggests ways of treating the many neurological and psychiatric disorders that result from mistakes in brain wiring.