The development of a Knowledge Based System (KBS) that can assist a designer in performing advanced multivariable control system design is the central theme of this paper. It starts by describing the role KBSs can play as designer's assistants. Then it is argued that the goal of developing a KBS can lead to a detailed analysis, and hence reformulation, of the essential design theory. The main body of the paper gives a description of how complex design concepts and strategies can be described and organised into a knowledge base. As the title suggests, this paper focuses on the actual implementation of a prototype KBS for control design: MAID. An example of a design session is included.link_to_subscribed_fulltex