Binary systems of stars are as common as single stars. They are of fundamental importance because they allow stellar masses, radii and luminosities to be measured directly, and explain a host of diverse and energetic phenomena including X-ray binaries, cataclysmic variables, novae, symbiotic stars, and some types of supernovae. This book is the first to provide a pedagogical and comprehensive introduction to binary stars. It combines theory and observations at all wavelengths to develop a unified understanding of binaries of all categories. It comprehensively reviews methods for calculating orbits, the Roche model, ideas about mass exchange and loss, methods for analysing light curves, the masses and dimensions of different binary systems, and imaging the surfaces of stars and accretion structures. This book provides a thorough introduction to the subject for advanced undergraduate and graduate students. Researchers will also find this to be an authoritative reference.