Simulation tools are widely used across the product, process and resource domains of product- and production development. This paper discusses the nature of simulation models and the wide use of simulation models .It uses virtual manufacturing, in particular discrete event simulation project methodology, as an example to elucidate important aspects of simulation, in particular human roles and some selected project phases of which verification and validation in relation to the simulation’s intended purpose are discussed in particular. The paper uses the NASA CAS model for credibility assessment of simulations to arrive at a schematic representation of how overall credibility as composed of aspect related to the model, the data, and the model’s use.