The development of the human brain, with its extraordinary range of cognitive abilities, is an awe-inspiring feat of evolution. Each of its tens of billions of cells must be born at precisely the right time, migrate to the correct locations, differentiate into as many as 3,000 distinct cell types, and form exquisitely specific synaptic connections with one another. Most of this happens before birth, but development continues for nearly three more decades. None of this is easy to study.