  Nuclear reactors are one of the most intense, pure, controllable,
cost-effective, and well-understood sources of neutrinos. Reactors have played
a major role in the study of neutrino oscillations, a phenomenon that indicates
that neutrinos have mass and that neutrino flavors are quantum mechanical
mixtures. Over the past several decades reactors were used in the discovery of
neutrinos, were crucial in solving the solar neutrino puzzle, and allowed the
determination of the smallest mixing angle $\theta_{13}$. In the near future,
reactors will help to determine the neutrino mass hierarchy and to solve the
puzzling issue of sterile neutrinos.
