Einstein, Podolsky, and Rosen (EPR) discovered an apparent paradox that seemed to unveil an inconsistency at the heart of quantum mechanics. It wasn’t until thirty years later that John Bell pointed out that there was a way to experimentally test the assumptions of the EPR argument and to distinguish between their deterministic version of reality and quantum mechanics. A long succession of experiments done since the 1980s have conclusively come down on the side of quantum mechanics and seem to have shown that the EPR assumption of local realism cannot be maintained. In this chapter, we give an introduction to these developments, which provide a theoretical underpinning to applications such as quantum cryptography, quantum secret sharing, and some quantum computing algorithms.