Model-based computer simulation plays a central role in many domains of the natural sciences and engineering as well as, increasingly, in the Life Sciences and the Social Sciences. Examples are provided by Computational Physics, Computational Chemistry, Computational Biology and Computational Neurosciences. On the basis of mathematical models, the computer allows us to explore complex systems which could not be described otherwise, and whose experimental investigation would involve considerably higher costs. Iterative model building, in interaction with simulations, requires the development of sophisticated methods, concepts and tools for the entire research process. Modern working and research environments use simulations not only on high-performance computers, but on all types of computational architectures, even on moderately powerful desktop computers. Model building, implementation, verification and validation are the central topics of simulation science.