Finding the desired piece of information is difficult and time-consuming due to the inherently chaotic organisation of the Web. For this reason, information integration services are becoming increasingly important. The idea of such a service is to offer to a user a single point of access that provides him or her ex-actly with the information he or she is interested in. To achieve this goal, the service dynami-cally integrates and customises data from various data providers. However, integration services face serious technical problems.
Users only communicate with mediators, which use wrappers to access data sources.
To this end, mediators and wrappers are connected by declarative rules that semanti-cally describe the content of data sources. This decoupling supports the stability of interfaces and thus increases the maintainability of the overall system. This thesis discusses query-centred MBIS, i.e., MBIS in which mediators represent their domain through a schema. The core functionality of a mediator is to receive and answer user queries against its schema. The higher the expressiveness of this these rules, the more types of heterogeneity can be overcome declaratively.