This chapter gives a general description of simulation and its associated system of interest. In the context of virtual manufacturing, three domains can be distinguished; product domain, process domain and resource domain. Examples of simulation in these three domains are given, as well as some examples of simulation across these domains. Typical steps/phases in a simulation project are described, as well as common pitfalls. In industrial simulation projects, usually a number of stakeholders are involved with different maturity/experience in the field of simulation. It is described how such industrial simulation projects can be supported by a handbook, developed in close collaboration with a group of companies. As one example of advanced applications, simulation-based remote monitoring and diagnostics is described. The other example of advanced applications given in the paper is that of simulation-based optimisation. Many simulation tools and projects aim at providing decision support to a human decision maker. High level information fusion, a development originating from defence research, also aims at providing decisions support. A comparison between virtual manufacturing and information fusion reveals that a popular reference model for information fusion called JDL-model is very apt to serve as a reference model for virtual manufacturing.