Direct Simulation Monte Carlo (DSMC) is a particle-based simulation method for gas dynamics. The method can be viewed as either a simplified molecular dynamics (DSMC being several orders of magnitude faster) or as a Monte Carlo method for solving the time-dependent nonlinear Boltzmann equation. The DSMC method has been used successfully in the study of rarefied gas flows for several decades but has recently found new applications in chemistry and physics. After outlining the method, novel applications (micro channel and surface dynamics studies, hydrodynamic and chemical fluctuations) and recent extensions (dense gas and liquid-vapor dynamics) of the method are described.