In order to design, manufacture, and assemble products most effectively, it is necessary to evaluate the effects of the variation that will naturally occur. The optimal method of analyzing this variation is through the use of simulation. Variation Simulation Modeling (VSM) is a simulation technique which can be used to study the effects of variation. With VSM computer simulation, design tolerances, assembly tooling, and assembly sequencing can be evaluated and optimized during the design stage before commitment to tooling, thereby preventing problems later at the prototype stage and during production start-up. This paper describes VSM and what it can be used for, how VSM is applied, and illustrative examples of its application within General Motors.