The paper discusses the concept of Federated Database Systems (FDBS), which are collections of cooperating but autonomous component database systems (DBSSs). It defines a reference architecture for distributed database management systems from both system and schema perspectives, outlines various FDBS architectures, and provides a methodology for developing one of the popular architectures. The paper also addresses critical issues related to developing and operating an FDBS, including schema translation, access control, negotiation, schema integration, query formulation, command transformation, query processing and optimization, and global transaction management. The authors emphasize the importance of managing distribution, heterogeneity, and autonomy in FDBSs, and provide a taxonomy of multi-DBMS and FDBS systems to compare different architectures. The paper concludes with a discussion of future research and unresolved problems in the field.The paper discusses the concept of Federated Database Systems (FDBS), which are collections of cooperating but autonomous component database systems (DBSSs). It defines a reference architecture for distributed database management systems from both system and schema perspectives, outlines various FDBS architectures, and provides a methodology for developing one of the popular architectures. The paper also addresses critical issues related to developing and operating an FDBS, including schema translation, access control, negotiation, schema integration, query formulation, command transformation, query processing and optimization, and global transaction management. The authors emphasize the importance of managing distribution, heterogeneity, and autonomy in FDBSs, and provide a taxonomy of multi-DBMS and FDBS systems to compare different architectures. The paper concludes with a discussion of future research and unresolved problems in the field.