This fourth edition of the brilliant work on quantum optics has been much updated. It now includes a new chapter on quantum entanglement and quantum information, as well as added discussions of the quantum beam splitter, electromagnetically induced transparency, slow light and the input-output formalism. The book still gives a self-contained and broad coverage of the basic elements necessary to understand and carry out research in laser physics and quantum optics, including a review of basic quantum mechanics and pedagogical introductions to system-reservoir interactions and to second quantization. The text reveals the close connection between many seemingly unrelated topics, such as probe absorption, four-wave mixing, optical instabilities, resonance fluorescence and squeezing. It also includes discussions of cavity quantum electrodynamics and atom optics. The extra material on the input-output formalism is needed to understand many problems in quantum optics.