The aim of this study is to describe the behaviour of the pressure in the Common Rail Diesel Injection System mathematically. In order to understand the wave phenomena that may occur in the system, a physical model is desired. The model will be used for examining the cause of problems in the OM613 Common Rail Diesel Injection Engine that arise at certain critical working conditions. Another object of a model of the system is to use it together with a model of the injector for diagnosis purposes. Two different modelling methods are used and both models are based on well known physical relations. The first approach implies that the pressure waves are approximated with mechanical waves in a mass spring system. The model developed by this method does not describe the measured data very well, which mainly depend on too inaccurate estimations of physical parameters. The second method is developed from the general wave equation. This model describes the system more strictly and presents accordingly much better results than the first model. For the above explained purposes the latter model is recommended. Simulations show satisfactory results but improvements are naturally possible. Since the models are developed for a certain working point they can not be expected to be valid for all working conditions. When the physical parameters for the critical working point becomes clear, a new model can be generated out of the ¯first one by only correcting a few constants.