In this contribution a method and its application for the integration of hydraulic systems into valve train simulation of combustion engines is introduced. Therefore the equations of multibody simulation models are augmented to enable a framework for the description of multibody-dynamics and hydraulics regarding constraints in and between the physical domains. Thus, stiff differential equations can be avoided, while the global dynamics of the systems are closely represented. Furthermore, the parametrization of the model is facilitated, as some parameters, such as contact damping and oil compressibilities, which are difficult to identify, are not necessary anymore for the model description. Detailed models for a hydraulic cam phaser and valve-clearance compensation are presented and their application is demonstrated in a valve-train simulation model.