Quantum optics, i.e. the interaction of individual photons with matter, began with the discoveries of Planck and Einstein, but in recent years it has expanded beyond pure physics to become an important driving force for technological innovation. This book serves the broader readership growing out of this development by starting with an elementary description of the underlying physics and then building up a more advanced treatment. The reader is led from the quantum theory of the simple harmonic oscillator to the application of entangled states to quantum information processing. An equally important feature of the text is a strong emphasis on experimental methods. Primary photon detection, heterodyne and homodyne techniques, spontaneous down-conversion, and quantum tomography are discussed, together with important experiments. These experimental and theoretical considerations come together in the chapters describing quantum cryptography, quantum communications, and quantum computing.