The volume presents an international collection of mature expert system projects. Work from countries ranging from Japan to France, Spain to China, and from the United States is reported. Following on the second stage of expert system development, discussion focusses on issues that have become prominent through the application of systems to real-world problems. The incorporation of substantial numeric knowledge into systems is the first issue covered. It arises in many technical domains, which are likely to have already been subject to statistical or algebraic description. The second issue concerns the explicit representation of control knowledge within knowledge-bases. This gives systems the capability of explicitly reasoning about the control strategy applicable to each sub-problem arising in a complex problem domain.