Most previous texts on quantum optics have been written primarily for the graduate student market at PhD level and above. Quantum optics: an introduction aims to introduce a wide range of topics at a lower level suitable for advanced undergraduate and masters level students in physics. The text is divided into four main parts, covering modern topics in both pure and applied quantum optics: I Introduction and background material. II. Photons. III. Atom-photon interactions. IV. Quantum information processing. The emphasis of the subject development is on intuitive physical understanding rather than mathematical arguments, although many derivations are included where appropriate. The text includes numerous illustrations, with a particular emphasis on the experimental observations of quantum optical phenomena. Each chapter includes worked examples, together with 10-15 exercises with solutions. Six appendices are included to supplement the main subject material.