The first round of the EinsteinBohr debates took place when Einstein challenged Bohrs principle of complementarity at the Solvay conference in 1927 and Bohr successfully defended it. The most serious challenge, however, came in 1935 when a paper by Einstein, Podolsky, and Rosen (EPR) argued that quantum mechanics was incomplete through a
gedankenexperiment motivating an approach based on hidden variables. In this chapter, EPRs arguments about the incompleteness of quantum mechanics and Bohrs reply to them are presented. The ultimate answer came almost 30 years later, almost ten years after Einsteins death, and was nothing that Einstein would have expected. Bells inequality and the subsequent Bell-CHSH inequality, that are satisfied by all theories based on the self-evident truths of reality and locality are discussed. The startling results that quantum mechanics violates Bells inequality and the experimental results are in agreement with the prediction of quantum mechanics are presented.