Expert systems are computer programs that rely on a body knowledge to perform a difficult task usually performed only by a human expert. This technology has been used in a number of subject areas to tackle a variety of problem types or roles, including interpretation, prescription, design, planning, monitoring and control, diagnosis and instruction. All eight of the roles considered applicable for expert systems have been used within environmental management, their uptake being reflected by the similarity to human cognitive process and the technological difficulties involved in their construction. There has also been a large in the number of expert systems that tackle more than one of the above roles. The continued expansion in numbers of expert systems is because of the increase in a single-role systems. Explanations of this trend are not certain but could be due to new researchers being drawn into the field and also the appearance of the first fully-working systems, some of which are now being marketed.