Security is important for Centralized Data Base Management Systems (CDBMS) and becomes crucial for Distributed Data Base Management Systems (DDBMS) when different organizations share information. Secure cooperation can be achieved only if each participating organization is assured that the data it makes available will not be abused by other users. In this work differences between CDBMS and DDBMS that characterize the nature of the protection problem in DDBMS are identified. These differences are translated into basic protection requirements. Policies that a distributed data base management protection system should allow are described. The system proposed in this work is powerful enough to satisfy the stated requirements and allow for variations on the policies. This system is a hybrid one where both authorizations and constraints can be defined. The system is termed hybrid because it combines features of both open and closed protection systems. In addition the hybrid system, although designed to offer the flexibility of discretionary systems, incorporates the flow control of information between users, a feature found only in some nondiscretionary systems. Furthermore, the proposed system is said to be integrated because authorizations and constraints can be defined on any of the data bases supported by the system including the data bases containing the authorizations, and the constraints themselves. The hybrid system is incorporated in a general model of DDBMS protection. A modular approach is taken for the design of the model. This approach allows us to represent the different options for the model depending on the set of policy choices taken. Three levels of abstraction describing different aspects of DDBMS protection problems are defined. The conceptual level describes the protection control of the DDBMS transactions and information flows. The logical level is concerned with the interaction between the different organizations participating in the DDBMS. The physical level is involved with the architectural implementation of the logical level. / Ph. D.