BESIII at the BEPCII accelerator is a major upgrade of BESII at the BEPC for the studies of hadron physics and τ-charm physics with the highest accuracy achieved until now. The peak luminosity of the double-ring e⁺e^？ collider, BEPCII, is 10³³cm^？2s^？1 at center-of-mass energy 3.78 GeV.
           The cylindrical core of the BESIII detector consists of a helium-based multilayer drift chamber (MDC), a plastic scintillator time-of-flight system (TOF), and a CsI(Tl) electromagnetic calorimeter (EMC), which are all enclosed in a superconducting solenoidal magnet providing a 1.0 T (0.9 T in 2012) magnetic field. The solenoid is supported by an octagonal flux-return yoke with resistive plate counter muon identifier modules interleaved with steel. The acceptance of charged particles and photons is 93% over 4 pi   solid angle. The charged-particle momentum resolution at 1 GeV/c is 0.5%, and the dE/dx resolution is 6% for the electrons from Bhabha scattering. The EMC measures photon energies with a resolution of 2.5% (5%) at 1 GeV in the barrel (end cap) region. The time resolution of the TOF barrel part is 68 ps, while that of the end cap part is 110 ps. The end cap TOF system is upgraded in 2015 with multi-gap resistive plate chamber technology, providing a time resolution of 60 ps.
           The BESIII Collaboration has about 600 members from 85 institutions in 17 countries (Institutes).