Quantum mechanics is more than 70 years old and forms the basis of our description of phenomena at the atomic level. Still, its internal structure and many aspects of its interpretation have been discussed ever since its conception. It is only during the last 10–15 years that novel experimental techniques have allowed physicists to carry out tests of a type which the quantum theory pioneers only dreamt of — the so-called “Gedanken experiments”.The present book shows how such crucial tests have led to a deeper understanding of basic quantum concepts and processes. This field is now developing rapidly, with important consequences for such diverse applications as measurement technology and quantum-based information transfer and computing.