This book is devoted to the modern problems of quantum optics. Atomic radiations in free space, waveguides, cavities and periodical structures are investigated. The wavepacket quantum-mechanical picture of radiation is stressed and different quantum states of the radiation field are described. The analysis of the appearance of coherence in lasers is followed by a discussion of the connection between the form of the Hamiltonian and the initial conditions in the radiation problems. The quantum theory of interference is also given and causality in the radiation process is investigated, and the resolvent method for radiation problems is also described. A new type of laser generator with an extremely narrow spectrum is then proposed. Finally, problems of the quantum-mechanical description of macroscopic body movement are discussed in connection with the observation of squeezed light.