The direct detection of gravitational waves (GWs) represents the opening of a new window into the universe. The existence of gravitational waves had been known from the observations of binary pulsars, which provide complementary precision tests of theories of gravity. These tests probe aspects like the curvature of space time, gravitational redshift, relativistic spin-orbit coupling, or fundamental principles like the universality of free fall. The timing of an array of millisecond pulsars also offers the opportunity to detect lowfrequency GWs directly. Sources to be detected include binary supermassive black holes, while future observations may also allow to measure the spin of gravitons and to constrain their mass. The talk will review recent observations and will explain how they complement GW observations.