The Daya Bay experiment, a collaboration of scientists from the United States, China, and Europe, measures how anti-neutrinos change from one flavor to another (electron neutrinos into muon or tau neutrinos). Three experimental halls, near a nuclear power plant that supplies power to Hong Kong and Shenzen, have water pools that contain anti-neutrino detectors (large tanks with transparent vessels filled with special liquids and instrumentation). Each pool also contains instrumentation that detects and cancels cosmic background radiation from the sun, atmosphere, and surrounding rock. The number of neutrinos measured in the two halls nearest to the reactors are compared to the number of neutrinos measured in the hall approximately two kilometers away to determine how many neutrinos are “disappearing” or changing. This measurement enables the collaboration to determine the last unknown neutrino mixing angle ?13 and make progress towards understanding the predominance of matter in the Universe.