The Synthesis Explorer system specializes in challenging problems of reactivity in organic chemistry, particularly reaction product prediction, mechanism prediction, and multi-step synthesis design. From our experience, the system's support of a mix of random and pre-constructed problems works best with random problems for extensive practice and pre-constructed problems for targeted testing and evaluation. The system is built on an underlying expert system whose inherent predictive power enables a richer learning experience through experimentation and interactive dialogue with customized feedback in the form of both text and predicted chemical structures. The majority of reagents covered in a second-year organic chemistry curriculum are already modeled within the system, and new reagents and reaction mechanisms are being added periodically. Mechanism prediction problems have recently been added alongside the existing synthesis design problems. Other features that could be pursued in the future include the capability to automatically assess the student's abilities and dynamically tailor the problems to fit the student's particular strengths and weaknesses. The expert system upon which Synthesis Explorer is built has been presented in an educational setting, to facilitate the learning of chemistry in ways previously unrealized. But the same expert system can be used in other applications in chemical informatics and modeling. For example, computerized retro-synthesis decision support systems (21, 22) could be based on the same technology. In fact, the underlying expert system is already being used to help solve the very kinds of synthesis problems generated by Synthesis Explorer (23). As the content and robustness of chemical expert systems expand, these will become useful not only to undergraduate students, but also to professional chemists. © Division of Chemical Education.