The design of hydraulic control systems is a complex and time-consuming task that, at the moment, cannot be automated completely. Nevertheless, important design subtasks like simulation or control concept selection can be efficiently supported by a computer. Prerequisite for a successful support is a well-founded analysis of a hydraulic system's structure. The paper in hand contributes right here. It provides a systematics for analyzing a hydraulic system at different structural levels and illustrates how structural information can be used within the design process. A further central matter of this paper is the automatic extraction of structural information from a circuit diagram by means of graph-theoretical investigations.