The use of computer simulation of vehicle dynamics as a development tool has come into its own over the past few decades. “Simulated” testing on a computer makes possible a degree of control and repeatability that allows the automotive engineer to determine the influence of design variables on different aspects of dynamic performance in ways that would be difficult or impossible by experimental methods.One of the software tools receiving wide acceptance for simulating trucks and combination vehicles is Truck-Sim™. The attraction of this program arises in part from its foundation of truck modeling methods developed at the University of Michigan Transportation Research Institute over the past two decades, and the use of an advanced graphical user interface to make the software both easy to understand and easy to use by design and development engineers.The paper provides an overview of the graphical user interface by which the user defines the vehicle and its sub-system properties, the control inputs (maneuvers), and the roadway environment. The interface also provides convenient means to evaluate vehicle performance by means of animations, and time history plots of engineering variables of interest. Examples are given showing how simulation can be used to identify design changes that can improve truck dynamic performance.