Knowledge representation is the basis of every knowledge-based system and hence, it is very crucial for the success of medical decision-support systems, which are special kinds of medical knowledge-based systems. Currently medical informational resources are written by the human expert and represented in a form which is sympathetic to the human expert. In order to a computer stores and manipulates medical knowledge, it must be represented in a logical form. This task is where medical knowledge representation comes in. In addition, medical knowledge representation is a main part of the solution to the problem of medical diagnosis. Therefore, the aim of this chapter is to provide a medical knowledge representation method to fulfill our needs.