The modelling of knowledge requires a methodology that goes beyond a simple use of technology.In a more general way, we can say that expert computer systems model something observed or postulated in an intelligent system. In other words, there is either the knowledge or the mechanisms that knowledge can emerge or a combination of both. The difference between AI applied and non-applied AI is not always essential. The Applied AI is not trying to be true to the knowledge of psychology and does not necessarily try to be smart. As mentioned, the main criterion is that of performance. The book exemplifies a modern AI Based Medical Expert system