In order to make model-driven product line engineering a reality, variability and feature modeling techniques are necessary, but not sufficient. Adequate tool support is of equal importance, and presents its own set of challenges. In this paper, we present an approach to the activities of modeling product lines, defining and verifying products, and exporting product models. Our approach pioneers innovations such as the active product view and an efficient decision propagation logic, and supports advanced concepts such as incomplete product definitions. We have implemented our approach on top of a UML tool.