Binary stars are dynamical systems formed by two stars that are physically bound by the gravitational force. Binary stars are privileged laboratories, allowing one to measure the fundamental properties of stars but also potentially changing the way stars live and die. Because of this, binary stars have continuously played a central role in astrophysics. In this chapter, we focus on the observational properties of binary stars. What are the fundamental quantities that describe binaries? How do we detect and classify them? Which parameters can we constrain for different type of objects?

This is a pre-print of a chapter for the Encyclopedia of Astrophysics (edited by I. Mandel, section editor F.R.N. Schneider) to be published by Elsevier as a Reference Module