  CMS has developed a fast detector simulation package, which serves as a fast
and reliable alternative to the detailed GEANT4-based (full) simulation, and
enables efficient simulation of large numbers of standard model and new physics
events. Fast simulation becomes particularly important with the current
increase in the LHC luminosity. Here, I will discuss the basic principles
behind the CMS fast simulation framework, and how they are implemented in the
different detector components in order to simulate and reconstruct sufficiently
accurate physics objects for analysis. I will focus on recent developments in
tracking and geometry interface, which improve the flexibility and emulation
performance of the framework, and allow a better synchronization with the full
simulation. I will then show how these developments have led to an improved
agreement of basic analysis objects and event variables between fast and full
simulation.
