In this monograph, the author combined a large number of similar problems encountered in various fields of modern quantum physics and having common features when considering multilevel quantum systems. He considers from a single point of view many different models and approaches developed in atomic, molecular, chemical, laser, nuclear physics, as well as in condensed matter physics. The book was based on lectures that the author gave at the Moscow Institute of Physics and Technology and at the Aime Cotton laboratory at CNRS (National Center for Scientific Research, France).