In previous chapters, we have seen how the new ideas in Einstein's gravity make small but striking corrections to the predictions of Newton's gravity, bending light more strongly as it passes the Sun and causing the orbits of planets to precess. Working out these corrections helped to ease us into the theory, to see that relativistic gravity is a natural development from Newtonian gravity. But the real excitement in modern astronomy and theoretical physics is in situations where Newtonian gravity doesn't even come close to being right. The Universe demands that astronomers use general relativity to explain what they see, and the deepest questions of fundamental physics demand that physicists even go beyond general relativity to find their answers. In this chapter we open the door on the richness of modern gravity by studying our first example of really strong gravitational fields: neutron stars.