The world is a seriously bizarre place: things can exist in two places at once and travel backwards and forwards in time, waves and particles are one and the same, and objects change their behavior according to whether they are being watched. This is not some alternative universe but the realm of the very small, where quantum mechanics rules. In this guide for the layman, the editors of New Scientist explain how quantum mechanics has given us many practical technologies—including lasers and the transistors that underlie computers and all digital technology—while looking to the future, suggesting that it may help us to arrive at a new understanding of the Big Bang and the nature of space and time.