Observational evidence for binary black holes and active double nuclei

I review the observational evidence for the presence of supermassive binary black holes in galaxies and for the presence of active pairs of galaxies. According to hierarchical galaxy merger models, binary black holes should form frequently, and should be common in the cores of galaxies. The presence of massive black hole binaries has been invoked to explain a number of class properties of different types of galaxies, and in triggering various forms of activity. Coalescing massive black hole binaries are powerful emitters of gravitational waves. The search for such binary black holes is therefore of great interest for several topics in astrophysics ranging from galaxy formation to activity in galaxies.