Monte Carlo simulations of vacuum in accelerators Marton Ady (CERN) Particle accelerators are clean environments, where residual gas can cause various problems. To minimize these, vacuum specialists are keeping the machines at very low pressures. In the Large Hadron Collider, for example, the beam pipes are kept below 10hoch-13 bars, which is as low as the pressure on the surface of the moon. At such low pressures, gases behave differently: since there is no interaction between the particles, fluid dynamics m odels developed for common conditions aren’t valid anymore. The aim of this seminar is to give an overview of techniques that can be applied in such low pressures, with focus on a simulator developed at CERN, that uses the test particle Monte Carlo method. The lecture walks through application examples and gives a hint at miscellaneous fields outside of the accelerator world where Monte Carlo methods can come to our help.