Federated database systems provide a uniform and transparent interface to distributed and heterogeneous databases. This interface consists of a federated database schema which is the result of a schema integration process. The federated database schema should correctly reflect the semantics of the component database schemata of which it is composed. Since the semantics of a database schema is also determined by a set of semantic integrity constraints, a correct schema integration has to deal with integrity constraints existing in the different component database schemata. This book investigates the role of integrity constraints in federated database schemata. In particular, it addresses the problem of relating local integrity constraints as a basis for the schema comparison process, the consistent definition and derivation of global extensional assertions, and the correct integration of existing local integrity constraints.