The Fast Simulation of the CMS Experiment

The CMS collaboration has developed a fast Monte Carlo simulation of the CMSdetector with event production rates $\sim$100 times faster than the \GEANTfour-based simulation (``full simulation),with nonetheless comparable accuracy for most of the physics objects typically consideredin the analyses. We discuss basic technical principles of the CMS Fast Simulation andtheir implementation in the different components of the detector, and we illustrate the most recent developments towards a tighter integration with the full simulation and a better flexibility.