The purpose of this book is to provide a physical understanding of what photons are and of their properties and applications. Special emphasis is made in the text on photon pairs produced in spontaneous parametric down-conversion, which exhibit intrinsically quantum-mechanical correlations known as entanglement, and which extend over manifestly macroscopic distances. Such photon pairs are well suited to the physical realization of Einstein-Podolsky-Rosen-type experiments, and also make possible such exciting techniques as quantum cryptography and teleportation. In addition, non-classical properties of light, such as photon antibunching and squeezing, as well as quantum phase measurement and optical tomography, are discussed. The author describes relevant experiments and elucidates the physical ideas behind them. This book will be of interest to undergraduates and graduate students studying optics, and to any physicist with an interest in the mysteries of the photon and exciting modern work in quantum cryptography and teleportation.