The articles collected in this volume were written for a Colloquium on Fifty Years of Quantum Mechanics which was held at the University Louis Pasteur of Strasbourg on May 2-4, 1974, in commemoration of the original work by De Broglie in 1924. It is our hope that this volume will convey to the reader the idea that quantum mechanics, besides being a fundamental tool for scien tific workers today, is also a source of a number of questions and thoughts about the interpretation of the foundation of quantum mechanics itself. This gives rise to problems of a philosophical and logical character and...