Design for assembly (DFA) involves the consideration of the assembly process while designing. It is important in that it is at the design stage that significant decisions about the assembly process are made. This paper overviews the background to DFA and presents an approach to DFA that operates by analyzing a design using constraint networks. This approach gives advice to the designer on changes that should be made to improve the design. The designer can interact with the constraint network system to make gradual improvements. The approach has been implemented as a prototype using the constraint language SPARK. The use of constraint networks for DFA has a number of advantages over more conventional techniques: advice is given on how to improve the design, the designer can interact with the DFA system to improve the design, and the designer can approach the problem from any viewpoint in that a rigid procedure does not have to be followed. The result is a flexible DFA system that can be adapted relatively easily to a particular environment.