The Jiangmen Underground Neutrino Observatory (JUNO) is an experiment under construction in China with the primary goal of determining the neutrino mass hierarchy (MH) with reactor anti-neutrinos. The JUNO detector system consists of a central detector, an active veto system and a calibration system. The central detector is a 35 meter diameter transparent acrylic sphere containing a 20 kton liquid scintillator neutrino target. A primary photodetection system consisting of 18 000 large (20'' diameter) dynode and microchannel plate photomultipliers surrounds the central detector. A second interlaced photodetection system is made of 25 000 small (3'' diameter) photomultipliers working in the single photoelectron regime for the reactor antineutrino detection. The detector is designed to achieve an unprecedented energy resolution of 3% @1MeV and an absolute energy scale uncertainty better than 1%. A veto system, consisting of a water Cherenkov detector and a top tracker, is used to to help maximally remove cosmogenic backgrounds. Due to its unprecedented scale and precision, JUNO will be an exceptional multipurpose detector with a rich physics program in neutrino oscillation, geo-neutrinos, astrophysical neutrinos and the search for physics beyond the Standard Model (sterile neutrinos, dark matter, proton decay and others).