The direct simulation Monte Carlo (or DSMC) method has, in recent years, become widely used for engineering and scientific studes of gas glows that involve low densities or very small physical dimensions. This method is a direct physical simulation of the motion of representative molecules,rather than a numverical solution of the equations that provide a mathematical model of the flow. the computations are nolonger expensive and the period since the 1976 publication of the original Molecular Gas Dynamics has seen enormous improvements in the molecular models, the procedures, and theimplementation strategies for the DSMC method. the molecular theory of gas flows is developed from first principles and is extended to cover the new models and procedures. For more information visit the website on http://ourworld.compuserve.com/homepages/gabird/