Binary stars are pairs of stars that are gravitationally bound, providing in some cases accurate measurements of their masses and radii. As such, they serve as excellent testbeds for the theory of stellar structure and evolution. Moreover, binary stars that orbit each other at a sufficiently small distance will interact during their lifetimes, leading to a multitude of different evolutionary pathways that are not present in single star evolution. Among other outcomes, this can lead to the production of stellar mergers, rejuvenated and chemically contaminated accreting stars, stars stripped of their hydrogen envelopes and gravitational wave sources. For stars massive enough to undergo a supernova, binary interaction is expected to impact the evolution of most of them, making the understanding of binary evolution a critical element to comprehend stellar populations and their impact at large scales.

Invited contribution to the encyclopedia of astrophysics (main editor I. Mandel, section editor F.R.N. Schneider). To be published by Elsevier