This revised new edition gives a unique and broad coverage of basic laser-related phenomena that allow graduate students, scientists and engineers to carry out research in quantum optics and laser physics. It covers quantization of the electromagnetic field, quantum theory of coherence, atom-field interaction models, resonance fluorescence, quantum theory of damping, laser theory using both the master equation and the Langevin theory, the correlated emission laser, input-output theory with applications to non-linear optics, quantum trajectories, quantum non-demolition measurements and generation of non-classical vibrational states of ions in a Paul trap. This fourth edition provides a new chapter on weak measurement, as well as a new chapter on complementarity. There is also new material included for atom optics and new problems have been added. Each topic is presented in a unified and didactic manner, and is accompanied by specific problems and hints to solutions to deepen the knowledge.