Computational cosmology is already a central part of modern, astronomical research, but continues to increase in importance. A small number of simple physical laws can give rise to an amazingly rich phenomenology, as numerous observations demonstrate so beautifully. The continuing increase in the speed and memory of computers allows us to perform ever more detailed simulations of complex astronomical phenomena, which can then be used to create "virtual observations". Such simulations are invaluable for testing theoretical models, for constraining the physical parameters of such models, and for gaining insight into the relative importance of different physical processes. Numerical simulations are also widely used for the design of observational campaigns, for testing data analysis software, and for revealing observational biases. This workshop will bring together researchers who are already active in various ongoing collaborative simulation projects such as OWLS, GIMIC, Aquarius, Millennium and Millennium with gas, and researchers who would like to get involved. The aim is to provide the participants with an opportunity to discuss recent results and challenges, to join existing collaborations, and to develop ideas for future collaborative projects. The first part of the meeting will consist of plenary talks and discussions, while the second part will focus on hands-on sessions in which participants can work together in small groups.