<P><EM>Introduction to Close Binary Systems</EM> provides a comprehensive survey and guide to the fast-moving field of multiple, specifically binary, stars, with an up to date account of research around ''close'', i.e. interacting pairs. Such interactions allow direct quantification of stellar properties, opening up factual insights into basic building blocks of the Universe.</P><P>The book provides a much needed update for the seminal Close Binary Systems of Zdenek Kopal. Following a comparable plan, it presents relevant subject matter with an emphasis on building a framework of understanding to serve as a supporting resource for students and researchers. <BR> <BR>The text starts from a general historical background and progresses into the main theoretical ideas supporting our prima facie interpretation of observations. The central chapters explore further into these observational methods, arranged according to the classic subdivisions of astrometry, spectroscopy and