This book presents an overview of the most fundamental aspects of the theory that underlies the Relational Database Model. As such it is self-contained though experience with formal models and abstract data manipulating on the one hand and with the practical use of a relational system on the other hand can help the reader. Such experience will offer the reader a better understanding of and a motivation for the different concepts theories and results mentioned in the book. We have focussed on the most basic concepts and aspects of the relational model without trying to give a complete overview of the state of the art of database theory. Recently a lot of books on databases in general and on the relational model in particular have been published. Most of them describe the use of database systems. Some clarify how information has to be structured and organized before it can be used to build applications. Others help the user in writing down his applications or in finding tricky ways to optimize the running time or the necessary space. Another category of books treat more fundamental and more general aspects such as the description of the relational model independent of any implementation the decomposition in normal forms or the global design of distributed databases. Few however are the books that describe in a formal way some of the subjects mentioned above.