Albert Einstein's General Theory of Relativity predicted results that were so incredible that even he did not accept them: space is expanding from a Big Bang, space itself contains an energy that is pulling the Universe apart from within, and deep chasms of gravity called black holes actually exist. Astonishingly, all of these wild ideas are now known to be true. But now we need to build on Einstein's work to take the next step -- to study the underlying physics of the very phenomena that came out of his theories. NASA's Beyond Einstein program consists of a series of space missions, large and small, that push Einstein's theories to their limits, using increasingly more sensitive probes. The two flagship missions now in development, Constellation-X and LISA, will explore extremes of space, measuring X-rays and gravitational waves. The smaller missions, the Einstein probes, will target specific science questions such as "What is Dark Energy?" and "What powered the Big Bang?"