The Mu2e experiment is designed to search for the charged-lepton-flavor-violating
process, $\mu^-$ to a $e^-$, with unprecedented sensitivity. The single 105-MeV
electron that results from this process can be mimicked by cosmic-ray muons or their
products entering the detector. An active veto detector surrounding the apparatus
is used to detect incoming cosmic-ray muons. To reduce the backgrounds to the required
level it must have an average efficiency of 99.99\% and excellent hermeticity over a
large area. The detector consists of four layers of scintillator counters, each with
two embedded wavelength-shifting fibers, whose light is detected by silicon photomultipliers.
The design and expected performance of the cosmic ray veto detector will be described.