Predicted in the 1930s and discovered in the 1960s by X-ray and radio astronomers, neutron stars are now known to be the typical result of the evolution of a massive star. There should be nearly of a billion of them in our galaxy alone. Neutron stars have roughly ten km radii and can spin six hundred times in a second. They can also have magnetic fields over a million billion times stronger than the Earth’s magnetic field. A small fraction of these neutron stars create bright radio emission and they can be observed as periodic radio pulses and are called radio pulsars. Radio pulsars have turned out to be superb cosmic laboratories and to provide tools to explore gravity and its radiation.