Binary systems of stars are as common as single stars. Stars evolve primarily by nuclear reactions in their interiors, but a star with a binary companion can also have its evolution influenced by the companion. Multiple star systems can exist in a stable state for millions of years, but can ultimately become unstable as one star grows in radius until it engulfs another. This volume discusses the statistics of binary stars; the evolution of single stars; and several of the most important kinds of interaction between two (and even three or more) stars. A series of mathematical appendices provides a concise but complete account of the mathematics of these processes.