The purpose of this book is to provide a physical understanding of photons and their properties and applications. Special emphasis is given in the text to 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-Rosen-Podolsky type experiments, and also make possible such exciting techniques as quantum cryptography and teleportation.