The explosion of the number of web-based information sources has drastically increased the need for intelligent mediation tools to be implemented between the users and these information resources. These tools must overcome the limitations of the current search engines while making it possible for the users to submit more sophisticated queries than simple key words. On the other hand, we can incorporate brief replies coming from various sources to build total responses to the users' queries. This technique is based on the integration of heterogeneous information sources. The central problem of information sources integration resides on their heterogeneity on the level of their format, the design and/or the semantic aspect. It is not possible to change these existing sources in order to make them homogeneous (for example XML or others). But, building a mediator on the top of these sources can solve the problem and allows a to query an information system which is centralized and hom...