For years, the human brain has been compared to a computer—but it is a computer without a wiring diagram. Researchers simply do not know how the billions of neurons in the brain are connected to one another, and without this information they cannot fully understand how the brain’s structure gives rise to perception and behavior. Now, for the first time, scientists have found a way to track the connections between a single neuron and other cells—a discovery that could eventually lead to a 3-D map of the brain’s wiring.