The subject of binary stars is always discussed in introductory texts in astronomy and astrophysics. The usual prescription involves the distinctions between visual (or resolved) binaries and the spectroscopic and eclipsing binaries, as well as schematic examples of resolved orbits, radial-velocity curves, and light curves. Examples of interacting binaries are discussed, and there are artists' impressions of Roche-lobe-filling stars sending gas streams across to impact an accretion disc surrounding a black hole, with jets of ejected matter from the inner regions of a thick accretion disc interacting with the local interstellar medium. A brief discussion usually emphasizes the importance of binaries for the determination of stellar masses and other parameters and their central role in explaining the properties and evolutionary states of many unusual stellar objects, such as novae, symbiotic stars, and x-ray binaries.

I have assumed that the reader of this text has already benefited from an introductory course in astronomy, including a careful reading of one of the many excellent introductory texts currently available. The basic ideas of astrophysics, including stellar evolution and the essential ideas about binary stars, should be well understood. I have assumed also that the reader has studied physics and mathematics to a similar level. Beyond these assumptions, I have tried to write a text that will be readily understood by an intermediate-to-advanced-Ievel undergraduate in astrophysics who is interested in the more practical, observational, and data-analysis aspects of studies of close binary stars.