Alain Aspect’, John Clauser and Anton Zeilingers’s names are associated with his seminal contributions to the measurement of Bell inequalities. A key element of quantum mechanics is entanglement. For entangled states, such as those first introduced by Einstein, Rosen and Podolsky (EPR), quantum mechanics predicts strong correlations between measurements of two particles, which have interacted at an initial time, but are separated at the time of the measurement. EPR interpreted these correlations as in terms of "local realism". It was Bell's fundamental contribution to realize that his local realism implied a limit on the correlations, as expressed by his celebrated Bell inequalities. This has led to a series of proposals and then experiments, first by Clauser, but culminating in the experiment by Aspect and Zeilinger with time-switchable polarization analysers in delayed choice experiments with correlated pairs of photons, rejecting theories which at the same time are local and realistic. Such experiments have paved the way for quantum communication including Zeilinger’s teleportation, and in a more general setting for quantum information processing.