Neutron stars are the remnants of massive stars that exploded as supernovae. They pack more than the mass of the Sun into a sphere no larger than a medium-sized city, making them the densest objects in the Universe. Pulsars, neutron stars that emit beams of radio waves outward from the poles of their magnetic fields, are perhaps the most extraordinary physics laboratories in the Universe. Research on these extreme and exotic objects already has produced two Nobel Prizes.