This book covers the distributed database models, explains the concepts of these models, and presents the way these models are used to integrate the database systems. Also this book covers the scientific researches that work on distributed database systems integration, especially when these systems are running on heterogeneous environments. At last this book suggests a new approach for designing and integrating distributed database system. This book can be used as reference in distributed database systems hence it contains the scientific and the practical materials on distributed database systems component such as fragmentation, allocation, replication, distributed query processing, query optimization and concurrency control , also it can be used as a guideline for developing a distributed database application.