  Neutrino telescopes are opening new opportunities in observational high
energy astrophysics. In these detectors, atmospheric muons from primary cosmic
ray interactions in the atmosphere play an important role. They provide the
most abundant source of events for calibration and for testing the
reconstruction algorithms. On the other hand, they represent the major
background source. The simulation of a statistically significant number of
muons in large volume neutrino telescopes requires a big effort in terms of
computing time. Some parameterizations are currently available, but they do not
explicitly take into account the arrival of muons in bundles. A fast Monte
Carlo generator (MUPAGE) was developed to generate single and multiple
atmospheric muon events in underwater/ice neutrino telescopes. The code reduces
the computing time for the simulation of atmospheric muons significantly. The
event kinematics is produced on the surface of a user-defined cylinder,
virtually surrounding the detector volume. The flux of muon bundles at
different depths and zenith angles, the lateral spread and the energy spectrum
of the muons in the bundles are based on parametric formulas. These formulas
were obtained according to a specific primary cosmic ray flux model and
constrained by the measurements of the muon flux in the MACRO underground
experiment. Some MUPAGE applications are presented.
