Concepts are seen as building elements of more compositional information objects in medicine. Consequently, representation of medical concepts plays a critical role in any information system. The authors present some of the key problems which need solution, such as definition of medical domain, analysis of internal structure, different subsets of medical concepts, and the problem of the 'elementary' concepts. As a result of these considerations we can conclude that the domain of medical concepts can hardly be delimited and consists of different subsets. These subsets need different representation methods according to their different nature. Some of these subsets have a hierarchic structure. In those one can find sometimes multiple hierarchies. We suggest avoiding multiple hierarchies in concept systems by the introduction of new dimensions.