Abstract: Due to an increased awareness of potential ontology applications in industry, public administration and academia, a growing number of ontologies are created by different organizations and individuals. Furthermore, ontology users or developers not only use their own ontologies but also want to integrate or adapt other ontologies or even apply multiple ontologies to solve a problem. Ontology mapping is the solution for finding semantic correspondences between similar elements of different ontologies. This correspondence is required in fields like data integration, information transformation and information retrieval. Even though, a lot of mapping techniques are developed, the objectives are not fully achieved. This review study aims to discuss various categories of ontology mapping techniques, the mapping strategies, challenges and further research requirements.