Reverse engineering is an approach that is used in many domains like electrical engineering, computer sciences and many others. The main object of RE is the analysis of an existing product (that can be a software application, a mechanical product ...) in order to produce a copy and/or an improved release of this one. In this chapter, we focus on reverse engineering in the mechanical engineering domain. According to that domain, the subject is a physical part. It could be a hand-made prototype, an old mechanical part, a modified part or tool, and, sometimes, a product of a competitor. It means that reverse engineering is used by people during any stage of the product development process. Actually, the needs of these different actors are very different but the fact is that current reverse engineering methodologies often propose only one type of approaches. Today, reverse engineering methodologies propose mainly geometric approach. This one generally considers a 3D point cloud from a 3D digitisation and enables an expert to fit a meshed surface or free form surfaces on this point cloud. The resulting model is a geometric shell that can be used efficiently by some of the actors of the product development process. For example, a meshed model can be used for mechanical analysis purpose. As a matter of fact, in many other cases, such a geometric model is not enough and knowledge about the history of the product is needed in order to improve the virtual model rebuilding. Late commercial solutions such as GeomagicTM or RapidFormTM, for example or complete CAD application software such as CATIATM are very efficient as they add segmentation algorithms, sketchers and/or many other facilities to the original surface rebuilding tools. These kinds of applications enable to rebuild the geometry of the object as a set of functions (protrusion, revolution, sweep...), they enable to add colours and textures, they enable realistic kinematical animation and many other things... Because the geometry of a given product is the consequence of an important process, it is important to try to recover any evidence of its past life (including socio-economical aspects, the design intents of the former designer, its different uses...) from its geometry in order to produce a model of a good quality. Such a model can provide important possibilities of Reverse Engineering. It enables to restudy the product more efficiently than a geometrical model based on a mesh or on free form surfaces. This chapter proposes approaches and methodologies that illustrate the recovering of these evidences and the use of this knowledge as explained by figure 1. There are two parts in this chapter. The first one proposes the general approach. It deals with how to integrate knowledge about mechanical product. Knowledge about the historical and economical environments of the product has to be considered as well as functional aspect in order to obtain a useful digital mock up that enable to redesign these products. The second part of the chapter focuses on a particular aspect of this general approach. It deals with manufacturing and functional knowledge about the product in order to improve the rebuilt CAD model of this product. An original CAD model is feature based and these features are the consequence of this knowledge.