The human brain has 100 billion nerve cells and trillions of connections between them. Understanding the workings of such a complex and dynamic organ requires new tools and technologies. Materials scientists are developing probes to form gentle but sensitive and reliable interfaces to stimulate and record signals from thousands of individual neurons at once. Our engineers are developing ways to manipulate neural circuits with electricity, light, ultrasound and magnetic fields, and others are listening to the brain, interpreting the language of neural signals and using that language to drive robotic arms or to type on a computer. New tools will enable as yet unimagined discoveries and will allow us to repair and even to augment the human brain.