The aim of this communication is to describe how aiding-design tools can evaluate designed solutions to help users make the best choices, avoid design mistakes and reduce the design time-cycle. First, we will compare the two main methods for aiding design behaviour simulation tools and domain knowledge simulation tools-and look at their advantages and drawbacks. We will focus on tools based on knowledge because of their `interactivity' and for their ability to represent domain knowledge and show how they can be extended to evaluate designed solutions. We will then concentrate on an aiding-design tool based on constraints and see how a solution can be evaluated using an evaluation function. As such a tool has already been developed as part of a European project to help metallurgists design and evaluate heat treatment operations, we end with the presentation of a real example.