Why is quantum theory so difficult to understand? In this book, written for both undergraduate and graduate students of chemistry and physics, 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 1930s, 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 1980s. Alternative interpretations -- pilot waves, quantum gravity, consciousness, and many worlds -- are described in the closing chapter. `I can recommend it warmly. Baggott has a practised, informal, attractive style taht renders the potentially turgid digestible . . . , he gives a lucid, thoughtful, and helpful account of one of this century's great conundrums.' The Times Higher Education Supplement'Baggott is a chemist; he combines scepticism with an honest and painstaking effort to understand the message coming from the Copenhagen priesthood, the physicists Irwin Schrödinger, Niels Bohr and their colleagues. He tries, with refreshing modesty, to pass on the results of his labours ... Baggott has an entertaining style, with clever use of analogies and diagrams - and, above all, its realist honesty that shines like a beacon through the fog of contemporary mystical specualtions.' Trevor Marshall and Max Wallis, New Scientist'The book is clearly written and should be of interest to all scientists and mathematicians.' Aslib Book Guide, Vol 57. No. 10, October 1992'this reasonably priced and well produced book forms a valuable bridge from basic undergraduate knowledge to the real stuff' R. Crossley, Institute of Physics Journal`I can recommend it warmly. Baggott has a practised, informal, attractive style that renders the potentially turgid digestible . . . , he gives a lucid, thoughtful, and helpful account of one of this century's great conundrums.' The Times Higher Education Supplement'Baggott's strength lies precisely in his appreciation of the problem of making real measurements on microscopic quantum systems using macroscopic devices. Baggott has an entertaining style, with clever use of analogies and diagrams - and, above all, its realist honesty that shines like a beacon through the fog of contemporary mystical speculations. The final sentence is particularly worth noting: "If you find the theory difficult to understand, this is the theory's fault - not yours."' Trevor Marshall, University of Manchester and Max Wallis, University of Wales, Cardiff, New Scientist, October 1922 Jim Baggott, Environmental Officer, Lubricants Marketing, Shell International.