This is the third, revised and extended edition of the acknowledged "Lectures on Quantum Optics" by W. Vogel and D.-G. Welsch. It offers theoretical concepts of quantum optics, with special emphasis on current research trends. A unified concept of measurement-based nonclassicality and entanglement criteria and a unified approach to medium-assisted electromagnetic vacuum effects including Van der Waals and Casimir Forces are the main new topics that are included in the revised edition. The rigorous development of quantum optics in the context of quantum field theory and the attention to details makes the book valuable to graduate students as well as to researchers. Voices to the new edition: "There are many good books in this area, but this one really excels in terms of broad coverage, choice of topics, and precision. It is very useful as a textbook for a quantum optics course, and also as a general reference for researchers in quantum optics. ... Also, the new edition includes some subtle and fundamental material about non-classicality, medium-assisted electromagnetic vacuum effects, and leaky cavities, based on research developed by the authors." Prof. Luiz Davidovich, Rio de Janeiro