Once completed, thePierre Auger Observatorywill be the cosmic ray detector with the largest aperture(2×7000&hthinsp;km2&hthinsp;sr).The design of this observatory and, in the future, the data analysis depend, at least partially, on simulations of the response of the detector to an extended air shower. In the following, we present briefly thePierre Auger Observatoryand discuss simulations of the surface detector. As a concrete example we discuss the simulation of calibration and monitoring methods. ©1999 American Institute of Physics.