The design of PAC systems benefits from modelling and simulation. Within the scope of the Dutch PACS project several simulation studies have shown that simulation is a powerful tool to evaluate hardware architectures, image management strategies and clinical working methods before they are actually implemented. Besides using simulation techniques for obtaining insight into a PACS's performance and behaviour, they can also be applied to actively support the design of a PACS. In order to capture the full complexity of PAC systems in a simulation model, and to take full advantage of simulation as a design tool, we are currently developing new techniques for model construction. This paper summarizes the underlying principles of our modelling method, and describes our first experiences with it.