TAP-EXTRA is a modular software tool-kit that can enhance process monitoring systems with cooperative, explanatory capabilities plus a methodology for adding explanations to a software system. These capabilities include providing explanations of a system's reasoning and conclusions in order to increase user confidence, co-operative problem solving between user and system, and certain types of intelligent assistance to the user. It is very useful for process monitoring when the operator needs help to: assess the risk; point out (po entially dangerous) mismatches between the user's view of the situation and the reality; provide a synoptic overview of the situation. One significant application, called Aleph, is a monitoring and risk assessment system for flood control in the city of Bordeaux. It gives spontaneous explanations and responds to situations developing in real time. It has shown the general applicability of the technology in the area of process monitoring systems. The TAP-EXTRA project has shown that industries and utilities controlling large processes benefit from this approach (water industry, chemical industry, blast furnaces). There is a wide range of domains to which the tool could provide real benefits. TAP-EXTRA is based on knowledge engineering technology which, although appearing in some ways to be a straight forward branch of Information Technology, does in fact require special expertise and experience over and above normal programming skill to build successful applications. This is equally true of the tool which for example requires developers to master the use of formal representation languages and to have skills and experience in knowledge acquisition techniques as well as knowledge of the application domain. There is a distinction between the tool and the application. The tool must be considered in a similar way to a compiler or database development system in that purchase of the tool does not provide the solution.