  The study of binary stars is worth to undertake not only to learn more about
the properties of binaries as such, but also because binaries are multi-purpose
astrophysical tools. This paper reviews some of the ways this effective "tool"
can be used, focusing on fundamental parameter determination, tests of
theoretical models, and the recent contribution of binary stars to establish
the distance to the Magellanic Clouds, and therefore, the first rungs of the
cosmological distance ladder.
