Modelling and simulation is of crucial importance for the understanding of system dynamics. In aircraft, simulation has been strong in the area of flight control. Modelling and simulation of the hydraulic systems has also a long tradition. The rapid increase in computational power has now come to a point where complete modelling and simulation of all the sub systems in an aircraft is not far away. This means new challenges in dealing with very complex multi domain systems. Of all the systems in an aircraft fluid power systems is one of the most difficult to handle from a numerical point of view. They are characterised by difficulties such as discontinuities, very strong non-linearities, stiff differential equations and a high degree of complexity.A considerable effort has therefore been made to develop methods suitable for simulation of such systems. This means that it is possible to simulate fluid power systems much more efficiently than before, and that a lot of systems can even be simulated in real time. The distributed modelling technique used, also makes it possible to partition the system for parallel processing. This means that it is possible to simulate much larger systems within reasonable time. It also is a valid paradigm for multi domain system.The rapid development in simulation methods and the general increase in hardware performance imply that simulations can be run thousands of times faster than just a few years ago. As a consequence, there is no longer such need to use very economic, more or less analytical methods. This means that design methods based on different kinds of numerical optimisation for system design, are becoming much more important.