Expert systems are software systems that can successfully compare to human experts. Their purpose is mostly advisory. Besides, they give explanation and advice to human experts when performing certain tasks. They are intelligent information systems, and are capable to explain and justify their conclusions. Knowledge systems are smaller software systems, and are usually less successful than human experts. Main reasons for expert systems development in medicine are: need for justification of decisions, need for enhancing performances in many uncertain relations; need for explaining of decision making process etc. One of the reasons of developing knowledge-based systems was that conventional statistic formalisms have not provided satisfactory solutions in medical decision making (MDM). Also, today, the relations between cases and conclusions are not universally valid. So, few causes can provide the same conclusion. Besides, data are not necessarily absolutely accurate. The area of applying expert systems is very wide: diagnosis, prognosis, self-education, directing etc. Basic structure of expert system consists of: knowledge, data base, inferring mechanism, explaining mechanism and user-interface. Though, expert systems also have certain bad features: primarily, they are not physicians i.e. they can not examine a patient. Furthermore, expert system that is good for one certain area is often not good for another one. There are some cases, when these systems can confuse a physician and make him to make a wrong decision. This occurs very often in two specific cases: when the clinical situation is urgent; and when accuracy of clinical information is not definite.