The integration of heterogeneous and autonomous information sources is a requirement for the new type of cooperative information systems. In this paper we show the advantages of using a terminological system for integrating pre-existing relational databases. From the resulting integrated schema point of view, using · a terminological system allows for the definition of semantically richer integrated schema. From the integrated schema generation process point of view, the use of a terminological system permits the definition of a more consistent, broad and automatic process. Last, from the query processing point of view, terminological systems provide interesting features for incorporating semantic and caching query optimization techniques. The advantages are presented in detail for each main step of the integration process: translation, integration and query processing.