Recent developments in cosmology and particle physics have led to the remarkable realization that our universe - rather than being unique - could be just one of many universes. In this volume, a number of active and eminent researchers in the field - mainly cosmologists and particle physicists but also some philosophers describe these recent developments and assess their implications, thus providing, for the first time, a broad and insightful overview of the subject. The Universe or Multiverse Symposium was chaired by Paul Davies.