  This paper will discuss the design and construction of BESIII, which is
designed to study physics in the tau-charm energy region utilizing the new high
luminosity BEPCII double ring e+e- collider. The expected performance will be
given based on Monte Carlo simulations and results of cosmic ray and beam
tests. In BESIII, tracking and momentum measurements for charged particles are
made by a cylindrical multilayer drift chamber in a 1 T superconducting
solenoid. Charged particles are identified with a time-of-flight system based
on plastic scintillators in conjunction with dE/dx (energy loss per unit
pathlength) measurements in the drift chamber. Energies of electromagnetic
showers are measured by a CsI(Tl) crystal calorimeter located inside the
solenoid magnet. Muons are identified by arrays of resistive plate chambers in
the steel magnetic flux return. The level 1 trigger system, Data Acquisition
system and the event filter system based on networked computers will also be
described.
