The proposed book is a bridge between the foundations of quantum mechanics and the theory of open quantum systems. The treatment of open quantum systems is a rapidly growing subject, of interest to an ever-increasing number of scientists. The main focus is on overcoming the idealization of perfect isolation of a quantum system, so as to allow for a more realistic treatment and to provide the relevant background for the development of quantum technologies. The book is addressed to students and researchers in the field of open quantum systems and more generally quantum mechanics, a wide community of physicists, mathematicians, and chemists. It is devised both to enter the field and to prepare a series of lectures or a one-semester course on the topic.