Binary and multiple stellar systems are an essential part of stellar physics, since single stars like the Sun are shown to be in the minority. Binary stars provide the most tractable, calibration-free path to fundamental stellar parameters (masses, radii, luminosities and temperatures). They also have vital interactions with planetary science, stellar structure and evolution (as calibrators), high energy astrophysics, galactic chemical evolution by way of supernovae, and cosmology. Binary stars also serve as standard distance calibrators, where the precision in the determined distance does not depend on the distance itself. The intent is to include all relevant kinds of observations of individual systems and populations, statistical studies, both analytical and numerical calculations, and database maintenance.

The main science topics central to the Commission are:

• the full range of observational tools that reveal binary and multiple systems (astrometry, photometry, spectroscopy, polarimetry, structure of cluster HR diagrams, products of space missions including Kepler, Gaia and LSST), and the interface of these with concerns of other Divisions and Commissions;
• kinematics, and ultimately dynamics, of binary and multiple systems;
• improved codes for binary and multiple systems, stellar atmospheres, structure and evolution, yielding surface compositions, tracks in the HR diagram, temporal changes in system masses and separations, etc;
• improvements of existing binary and multiple star system databases, regular production of publications (online and perhaps in print on paper), sharing databases, new codes, exciting discoveries, and opportunities for interaction with other parts of the astronomical community; and
• accurate knowledge of the history of the subject and its continuing impact on astronomy in general.

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