In this paper we introduce a method for knowledge extraction in early stages of design by the interpretation of low-level CAD data, i.e. the geometry of mechanical products. Therefore, only the 3D CAD model of the product, including its components, is needed. The information about physical principles and functions is extracted automatically. First a more detailed introduction to the design process in mechanical engineering is presented. This enables the reader to understand the information which is accumulated during product design and which must be recognized during the interpretation process, i.e. the redesign. Then the approach of low-level CAD data interpretation is presented in detail. This includes the recognition of physical principles and product functions and the automatic construction of effective geometries and function structures. At the end the approach is verified by presenting an example of a product geometry, which is interpreted stepwise.