A symmetry of the fundamental (microscopic) equations of motion of a system; if it holds, the time reversal of any motion of the system is also a motion of the system. To date, only two phenomena have shown evidence (at least indirect) for violation of time reversal invariance. One is the violation of CP invariance observed in the decays of the neutral mesons K_L and B⁰, B⁰. The other is the baryon asymmetry of the universe.