The book is devoted to quantum measurement theory, experiencing another rise due to new applications to quantum information technology. An analysis of the phenomena of the decoherence of the measured system is given and a simple technique is developed to describe a wide class of quantum measurements, including the method of complex Hamiltonians for continuous measurements or continuous decoherence. The conceptual problems of quantum mechanics are also touched upon. The book is of interest to researchers, graduate students and senior students specializing in quantum mechanics, mathematical physics and the theory of stochastic processes.