The brain is the most complex organ we know. But how does this extraordinary network of cells—capable of thought, memory, and emotion—come into being and take shape? Gabriele Ciceri and Lorenzo Cingolani take us on a journey to discover the essential elements involved in building the brain: cells capable of acquiring different identities, signals that guide their development, connections that form in the right place, and processes that unfold with remarkable precision in time. All it takes is for this delicate balance to be disrupted for development to take a different course. This is precisely what happens in some diseases.