Quantum theory attempts to describe the discrete or atomic nature of matter and the physical world. Certain paradoxes connected with the use of our familiar ideas of the theory have led some physicists to suggest that a revision of quantum theory at its most fundamental level is now inevitable, while others think that the wide range of experimental success of the theory make such changes literally unthinkable. This book contains the edited papers presented at a small informal colloquium held in Cambridge in 1968 to discuss the need for fundamental revision in quantum theory. Most schools of thought on the foundations of the theory were represented, and to direct discussion some participants proposed actual changes. A principal aim was to pinpoint the source of difficulty in current ideas of the time or, failing that, to present alongside each other the various viewpoints about them.