  The Gamma-ray Large Area Space Telescope (GLAST) is now being designed by a
number of collaborating institutions. It will study the cosmic gamma radiation
from 20 MeV to 300 GeV with high precision and sensitivity, greatly expanding
on the important EGRET results. One of the key systems of the instrument, the
Anticoincidence Detector (ACD), is designed to reject the majority of charged
particles, which are the background for any gamma-ray experiment. The ACD of
EGRET has suffered from the self-veto effect when the products of the high
energy photon interactions in the instrument's calorimeter cause a veto signal
in the anticoincidence detector (backsplash effect), resulting in the
degradation of the efficiency for high energy (> 5 GeV) gamma rays. To avoid
this effect, the ACD for GLAST is divided into many scintillating tiles with
wave-shifting fiber readout. The design of this detector along with the beam
test and simulation results are given in this paper.
