This thesis presents a design for an Intelligent Educational System to support the teaching of design evaluation in engineering. The design consists of a simple computerbased tool (or 'learning environment') for displaying and manipulating infonnation used in the course of problem solving, with a separate dialogue component capable of discussing aspects of the problem and of the problem solving strategy with the user. Many of the novel features of the design have been incorporated in a prototype system called WOMBAT. The main focus of this research has been on the design of the dialogue component. The design of the dialogue component is based on ideas taken from recent work on rational agency. The dialogue component has expertise in engaging in dialogues which support collaborative problem solving (involving system and user) in domains characterised as justified beliefs. It is capable of negotiating about what to do next and about what beliefs to take into account in problem solving. The system acquires problem-related beliefs by applying a simple plausible reasoning mechanism to a database of possible beliefs. The dialogue proceeds by turn-taking in which the current speaker constructs their chosen utterance (which may consist of several propositions and questions) and explicitly indicates when they have finished. When it is the system's turn to make an utterance, it decides what to say based on its beliefs about the current situation and on the likely utility of the various possible responses which it considers appropriate in the circumstances. Two aspects of the problem solving have been fully implemented. These are the discussion about what criteria a decision should be based on and the discussion about what decision step should be taken next. The system's contributions to the interaction are opportunistic, in the sense that at a dialogue level the system does not try to plan beyond the current utterance, and at a problem solving level it does not plan beyond the next action. The results of a formative evaluation of WOMBAT, in which it was exposed to a number of engineering educators, indicate that it is capable of engaging in a coherent dialogue, and that the dialogue is seen to have a pedagogical purpose. Although the approach of reasoning about the next action opportunistically has not proved adequate at a problem solving level, at a dialogue level it yields good results.