The direct simulation Monte Carlo (or DSMC) method is widely used for the modeling of gas flows through the computation of the motion and collisions of representative molecules. Computation at the molecular level is necessary for studies in rarefied gas dynamics (or RGD) because the transport terms in the Navier-Stokes equations are not valid in this flow regime. The essential characteristic of a "rarefied" flow is that the molecular mean free path is not negligible. This means that many applications involve normal and high density flows with very small physical dimensions. At sufficiently high density, the DSMC fluctuations can be physically real and the method then goes beyond the mathematical model provided by the Boltzmann equation. In addition, the DSMC method is employed for the study of chemical reactions for which inverse collisions cannot be defined and the Boltzmann model is inapplicable.