Distributed Database Systems (DDBS) may be defined as integrated database systems composed of autonomous local databases, geographically distributed and interconnected by a computer network. The purpose of this monograph is to present DDBS concurrency control algorithms and their related performance issues. The most recent results have been taken into consideration. A detailed analysis and selection of these results has been made so as to include those which will promote applications and progress in the field. The application of the methods and algorithms presented is not limited to DDBSs but also relates to centralized database systems and to database machines which can often be considered as particular examples of DDBSs. The first part of the book is devoted to basic definitions and models: the distributed database model, the transaction model and the syntactic and semantic concurrency control models. The second discusses concurrency control methods in monoversion DDBSs: the locking method, the timestamp ordering method, the validation method and hybrid methods.