In this contribution, a modular modeling approach for a PEM-FC system is presented, which takes the demands for automotive application into account. Contrary to stationary applications or low-power fuel cell systems, challenges arise in particular by the increased supply system complexity, the importance of the overall transient behavior and the wide operating range. Initially, the need for a strict methodology is motivated, and a basic structure for a system design is proposed. The distinction between flow and effort elements allows a modular and intuitive system structure exemplary depicted for the cathode air supply. Due to the applied integration of the stack, the development of stack and subsystem model functionalities can be done separately. For the modeling of the cathode supply components, physical and phenomenological approaches are used and discussed individually in more detail. The evaluation is based on test bench data that indicates very good compliance. Oxygen supply and humidity, which are both critical operation parameters for the stack, can be calculated appropriately and proves the suitability of the model. Preliminarily to the controller design, the model can be used to analyze the non-linearities and the dynamic behavior of the cathode supply system. Furthermore, the application of control and diagnostic functions and the examination of operation strategies is possible. Current work focuses on sensitivity analysis to investigate the couplings between the components and parameter optimization to increase accuracy on the one hand and decrease identification effort in order to improved usability of the system model on the other hand. For this, the modular design seems to offer an ideal foundation able to handle the complexity.