Optics experiments allow wide ranging and important tests of quantum mechanics (QM). In conjuction with atomic physics, they are used to investigate such diverse questions as parity nonconservation and nonlinear extensions of quantum mechanics, issues whose proper place is many orders of magnitude above the 1-eV energy scale. Optics is also important in the study of gravitational phenomena, such as gravitational antennas and gravitational lensing by some of the most massive objects in the universe. The chapter explores that the two great revolutions in physics that occurred at the beginning of the twentieth century: quantum mechanics and relativity. The chapter illustrates that optical experiments probe deeply the understanding of these two branches of physics. The reason for this is that the photon is at once both a quanta1 and a relativistic particle. The conceptual tension between the fundamentally nonlocal nature of the quantum on the one hand and the intrinsically local nature of space-time on the other is most sharply manifested in optical phenomena and provides a strong motivation for pushing optical tests to new frontiers.