Radio pulsars are extremely compact neutron stars. Some of them have a rotational stability that is comparable to the precision of the best atomic clocks. In particular pulsars in binary systems are therefore ideal to test Einstein's general theory of relativity in strong gravitational fields. Changes in the orbital motion of binary pulsars are presently the only firm evidence for the existence of gravitational waves as predicted by general relativity. It is expected that in the next few years pulsars will allow the direct measurement of gravitational waves caused by super massive black holes.