  The mass hierarchy and the CP phase are the main focus of the next generation
neutrino oscillation experiments. Jiangmen Underground Neutrino Observatory
(JUNO), as a medium baseline reactor antineutrino experiment, can determine the
neutrino mass hierarchy independent of the CP phase. The physics potential on
the mass hierarchy, and other measurements are reviewed. The preliminary design
options for a 20~kton detector with an energy resolution of
$3\%/\sqrt{E_{vis}}$ are illustrated. The main technical challenges on the PMT
and scintillator are discussed and the corresponding R\&D efforts are
presented.
