  Binary systems have long been recognized as the source of powerful
astrophysical diagnostics. Among the many applications of binary stars, they
have been used as probes of stellar structure and evolution (both of single and
binary stars) in a broad range of masses, evolutionary stages, and chemical
compositions, and as indicators of distance and time. With the numerous ongoing
photometric surveys and upcoming space astrometry and photometry missions, the
future of binaries looks bright. The various aspects of binaries as
astrophysically useful laboratories are reviewed here, with emphasis on the
currently open problems and research opportunities.
