This chapter explains the roles of a data dictionary (DD) in the development of the information system (IS) and describes its components. The chapter presents a possible implementation of the data dictionary both with the relational and the object oriented (OO) models. A DD is a database or repository of data on the products of the analysis and design phases. It is initially created during the analysis phase, containing details about the components of the object oriented data flow diagram (OO-DFD) and the data elements carried by their various dataflows. But it will be updated and extended throughout the design phase to include details about the products of that phase too. The DD is essential in the development of large-scale systems because it enables its users (analysts and designers) to define, save, and retrieve, in a standard manner, various details of the analysis and design products, details which are not included in those products. A DD can be implemented as a relational database consisting of tables, or as an OO database consisting of object classes. This chapter describes the components of a DD using both models. Obviously, at this stage we only describe the components which are created at the analysis phase.