This thesis describes a formal methodology for defining and assessing product performance and its implementation in a prototype computer system. The methodology is based on abstract descriptions of the operations that are conducted within the design process. It is, consequently, extremely generic and creates a bridge between physical product performance and actual user requirements. The methodology is based on defining product attributes in terms of observable parameters of the product in use. Defining an attribute in this way inherently reflects its required interaction with the user and consequently can truly be said to be in "user terms" A product will have a range of attributes and a performance indicator is proposed, such that the attributes are combined in a way that reflects their relative importance to the user. At the conceptual stage of the design process, when the actual product does not exist, and only some abstract representation is available, it is vitally important to be able to model or simulate and hence evaluate the product attributes. This area of design has often been associated with non algorithmic design procedures, because of its intangible nature. In this thesis the attribute methodology has been used to implement a prototype Computer Aided Design Evaluation Tool (CADET), which has been used and tested with an existing product range. The methodology being abstractly defined supports a wide range of product attributes. It also gives an indication of how the correspondingly wide range of existing analysis software could be integrated into a powerful single Computer Aided Design system. This work has resulted in the publication of two papers in refereed Journals and the presentation of eight other papers at refereed International Conferences. A list of the publications is included in the Appendices.