Pulsars are fast spinning, highly magnetised neutron stars that emit beams of radio waves from their magnetic poles. Because of this, pulsars behave like cosmic lighthouses: once per rotation, if the emission cones cross the line of sight to the Earth, we observe a radio pulse. The fastest pulsars, spinning hundreds of times per second, have an extremely stable rotation and, if undisturbed, their signals arrive on earth like clockwork. If, however, something happens to the pulsar, or to the medium their signals travels through, or to the space-time between pulsar and Earth, the times of arrival of the radio pulses get affected in tiny but measurable ways. Thanks to this, pulsar can be used as probes in many different fields of astrophysics and fundamental physics: from the study of the equation of state of nuclear matter, to the study of the interstellar medium, to relativistic gravity etc. In this talk, after briefly introducing pulsars and the analysis techniques of their pulses times of arrival, I will discuss how they can be used as testbeds of general relativity and detectors of gravitational waves.