'In recent years multiverse theories have attracted a great deal of attention among physicists, promising simple resolutions to fundamental long-standing problems. Philosopher of science Simon Friederich provides here a valuable, careful examination of these claims and their relation to testable science. The problems posed by the advent of such theories that arguably cannot be shown to be wrong seem to be here to stay. Friederich's comprehensive and even-handed account of all sides of the question of where we are now with this new sort of science has an important role to play.' Peter Woit, Columbia University