One of the challenges of current medical information systems which is based on keyword searching, is that it may retrieve a large amount of irrelevant information during searching. Also, these systems don't provide interoperability among healthcare systems. For interfacing these challenges, and for the purposes of more interoperability between user and machine, semantic web (web 3) has been developed by Berners-Lee. One of the main components of semantic web is ontology, the application of which is a main step to make effective searching ininformation systems. This paper is intended to study the application of ontologies in medical and healthcare information systems.