After the discovery of neutrino oscillations, two angles of the leptonic unitary mixing matrix have been determined. However, for Θ13 only upper limits exist. The best constraints have been obtained in the reactor oscillation experiment CHOOZ. By reducing statistical as well as systematical uncertainties, the sensitivity on Θ13 of new reactor experiments can be improved by one order of magnitude. This article focuses on the proposed reactor experiment DOUBLE-CHOOZ.