In this book the author looks at the continuing debate about the meaning of quantum theory. The historical development of the theory is traced from the turn of the century through to the 1930's, and the famous debate between Niels Bohr and Albert Einstein. The book examines in detail the arguments that quantum theory is incomplete, as made by Einstein, Boris Podolsky, and Nathan Rosen; the development of Bell's theorem; and crucial experimental tests performed in the early 1980's. Alternative interpretations - pilot waves, quantum gravity, consciousness, many worlds, and God - are described in the closing chapter. This book is aimed at graduate and senior undergraduate students of physics and chemistry taking quantum chemistry or quantum theory courses, and other scientists interested in the subject.