This paper has provided examples of effective use of model-based design tools throughout the development of a hybrid electric vehicle architecture and control system. In the architecture selection phase, a novel technique was presented for evaluating different architectures based on the system degrees of freedom. This technique was used to show that a series parallel powertrain has the potential for better fuel economy over a series powertrain. Without the need to develop a control strategy drive cycle simulations gave fuel economy estimations. Control system architecture development was then overviewed. This step was important to model-based design because at this point the requirements of the control system and each controller in the system were well defined. These requirements directly drive the requirements and fidelity of models used to validate the control system.