This paper presents a functional system architecture for an “autonomous vehicle” in the sense of a modular building block system. It is developed in a top-down approach based on the definition of the functional requirements for an autonomous vehicle and explicitly combines perception-based and localization-based approaches. Both the definition and the functional system architecture consider the aspects operating by the human being, mission accomplishment, map data, localization, environmental and self-perception as well as cooperation. The functional system architecture is developed in the context of the research project “Stadtpilot” at the Technische Universität Braunschweig.