The semantic Web suggests to annotate Web resources with machine-processable metadata; and ontologies, as means to conceptualize and structure knowledge, are seen as the key to realize the vision of the semantic Web. However, ontologies themselves do not provide semantic interoperability, since a single ontology cannot be used to represent all kinds of domains and applications. Ontology mapping, therefore, is introduced to achieve knowledge sharing and semantic integration in an environment with different underlying ontologies. This chapter provides an overview of the approaches to information integration developed by researchers in the community of ontology mapping. After introducing the backgrounds and concepts of ontology mapping, a comprehensive and detailed treatment of different ontology mapping schemes is presented. Closely related aspects of ontology mapping, such as mapping result description, similarity measures, algorithm performance evaluation and so forth, have also been addressed. The emphasis is to present a snapshot of the fast-growing field in a manner suitable for both experts and non-specialists.