Structural representations for interfaces between modules and components in a product vary widely in the literature. After reviewing several structural approaches to interface definition, a new weighted design dependency measure is described. The new representation takes into account both six different types of interfaces as well as their relative strength and frequency within a product architecture. The resulting design dependency measure provides a means for designers to quantify the change resistance in a product. In this paper, we investigate the use of this new design dependency measure to drive module identification. Specifically, we compare the resulting modules obtained by optimizing Design Structure Matrices (DSMs) using standard 0-1 representations of the interfaces to those obtained using the new design dependency measure. The results indicate that the weighted design dependency measure leads to more a logical definition of modules that maximizes within module dependencies and minimizes interactions between modules.