Fig. 1: Expert systems are succesfully applied for tax and legal support.

An expert system, a system based on knowledge, is an instrument capable of solving problems within a limited domain, but with a performance similar to that of a human being expert in that domain.
This means that the main task of an expert system is to assist the activity of professionals, where it is usually necessary to consult a human specialist with expertise and intelligence. AI research has identified the problems arising in creating such instruments, and has found that it is necessary to restrict the field of application as much as possible. Therefore, compared to a human expert, these applications are more limited and superficial, lacking the completeness represented by a competent person's cultural knowledge.

The first and best known of these systems is Mycin, which E.M. Shortleffe started to develop in 1972, and which was applied in medicine.

With regard to the kind of problems which an expert system can solve we can list the following (obviously far from complete):

• "diagnosis": this identifies the possible causes of a malfunction by recognising a set of symptoms and suggesting possible treatment;
• "monitoring": this follows the development of a process through time; it controls the acquisition and elaboration of different kinds of data, providing the output of synthetic information on general conditions and estimates on the evolution of the process;
• "planning": once the resources/actions available are known, this identifies their best usage in order to reach a certain goal in a certain time; at the same time it presses for the acquisition of new resources; 
• "data and signal interpretation": with the input of a set of data referring to a certain issue, it is possible to make a general assessment in order to recognize the impending arrival of some predetermined situations.