Binary systems of stars are as common as single stars. This original text provides a pedagogical and comprehensive introduction to binary stars. The author combines theory and observations at all wavelengths to develop a unified understanding of binaries of all categories. Chapters review methods for calculating orbits, the Roche model, ideas about mass exchange and loss, methods for analyzing light curves, the masses and dimensions of different binary systems, and imaging the surfaces of stars and accretion structures. This volume offers advanced undergraduate and graduate students a thorough introduction to binary stars that will aid their learning of stellar astrophysics, stellar structure and evolution, and observational astrophysics.