The quantum measurement problem is one of the most fascinating and challenging topics in physics both theoretically and experimentally. It involves deep questions and the use of very sophisticated and elegant techniques. After analyzing the fundamental principles of quantum mechanics and of the Copenhagen interpretation, this book reviews the most important approaches to the measurement problem and rigorously reformulates the "collapse of the wave function" by measurement, as a dephasing process quantitatively characterized by an order parameter (called the decoherence parameter), according to the many-Hilbert-space approach to the problem.The book deals not only with the measurement processes (including imperfect measurements) but also with related interference and mesoscopic phenomena -- by means of general arguments -- of solvable models and of numerical simulations. The quantum Zeno effect and the issue of irreversibility are also discussed.