  Coalescing massive black hole binaries are powerful emitters of gravitational
waves, in the LISA sensitivity range for masses M_BH ~ 10^{4-7} M_sun.
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. The search for such binary black holes is therefore of great interest
for key topics in astrophysics ranging from galaxy formation to activity in
galaxies.
  A number of phenomena were attributed to the presence of supermassive binary
black holes, including X-shaped radio galaxies and double-double radio
galaxies, helical radio-jets, periodicities in the lightcurves of blazars,
(double-horned emission-line profiles), binary galaxies with radio-jet cores,
binary quasars, and the X-ray active binary black hole at the center of the
galaxy NGC 6240. Here, I review the observational evidence for the presence of
supermassive binary black holes in galaxies, and the scenarios which have been
discussed to explain these observations.
