This chapter tells the story of Albert Einstein's theory of relativity, the "greatest feat of human thinking about nature, the most amazing combination of philosophical penetration, physical intuition, and mathematical skill" as physicist Max Born described Einstein's achievement. Einstein's theory became quickly accepted, especially after a series of lectures on the mathematical formalism of special relativity that Minkowski gave in 1908. In early 1916, within months of the publication of Einstein's theory, physicist and astronomer, Karl Schwarzschild found a solution to the field equations of general relativity in the simple case of spherical symmetry. While general relativity could be tested in distant astronomical systems, space scientists wanted to probe general relativity in the solar system directly. Besides its enormous scientific potential and ongoing discoveries, Einsteinian physics and the emerging discipline of gravitational wave astronomy have the opportunity to motivate students and encourage interest in STEM topics.