Database management systems (DBMS) are becoming part of environments composed of large-scale distributed heterogeneous and networks of autonomous, loosely coupled components. In particular, federated database management systems (FDBMS) can be seen as networks that integrate a number of pre-existing autonomous DBMS which can be homogeneous or heterogeneous. They can use different underlying data models, data definition and manipulation facilities, transaction management, and concurrency control mechanisms. DBMS in the federation can be integrated by a mediator providing a unified view of data: a global schema, a global query language, a global catalogue, and a global transaction manager. The underlying transaction model considers, in general, a set of transactions synchronized by a global transaction. Synchronization is achieved using protocols such as the Two-Phase Commit protocol. FDBMS applications are built upon this global environment, and they interact with the mediator to execute global database operations (i.e., operations that can concern various DBMS in the federation).