Jets of extragalactic radio sources in many cases show signs of precessional motion. This may arise when two massive compact objects are close to each other within a galaxy nucleus. The observed radio brightness distributions are modelled by slowly precessing jets, often relativistic. Interesting illustrations of their behaviour are offered by the frequency of occurrence of one-sided jets and by an explanation of the "preferential avoidance'' effect. There may be some evidence for a binary massive black hole in the nearest galaxy nucleus, the Galactic Centre.