The chapter is devoted to General Relativity. The goal is to rigorously arrive at the equations that describe the structure of relativistic stars — the Oppenheimer–Volkoff equations —, the form that Einstein’s equations take for spherical static stars. Two important facts emerge immediately. No form of matter whatsoever can support a relativistic star above a certain mass called the limiting mass. Its value depends on the nature of matter but the existence of the limit does not. The implied fate of stars more massive than the limit is that either mass is lost in great quantity during the evolution of the star or it collapses to form a black hole.