Stars are mostly found in binary and multiple systems, as at least 50% of all solarlike stars have companions – a fraction that goes up to 100% for the most massive stars. Moreover, a large fraction of them will interact in some way or another: at least half of the binary systems containing solarlike stars, in particular when the primary will evolve on the Asymptotic Giant Branch and at least 70% of all massive stars. Such interactions can, and often will, alter the structure and evolution of both components in the system. This will, in turn, lead to the production of exotic objects whose existence cannot be explained by standard stellar evolution models. Moreover, the chapter explores one of the most luminous stars in our Galaxy, Eta Carinae. The year 2016 saw the first ever announcement of the detection of gravitational waves, coming from the merging of a binary black hole. In this chapter, the author leads the reader through a walk in the zoo of binary stars, highlighting some specific examples.

The statistical distributions of main-sequence multiple-star properties reveal invaluable insights into the processes of binary star formation, and they provide initial conditions for population synthesis studies of binary star evolution. Binary stars are discovered and characterised through a variety of techniques. Correcting for their respective selection effects and combining the bias-corrected results is not a trivial process. This is partially because the intrinsic distributions of companion frequency, primary mass M1, orbital period P, mass ratio q and eccentricity e are all interrelated , i.e., f(M1,P,q,e)/= f(M1)f(P)f(q)f(e). In particular, the binary fraction increases with primary mass, especially across short orbital periods, and binaries become weighted towards larger eccentricities and more extreme mass ratios with increasing separation, especially for more massive primaries. Moreover, binary star statistics vary with age, environment and metallicity. This chapter summarises the strengths and limitations of the various observational techniques, and reviews the statistical correlations in the intrinsic (bias-corrected) multiple-star properties.