This volume presents the proceedings of the 1991 Symposium on Mathematical Fundamentals of Database and Knowledge Base Systems, held in Rostock, FRG, May 6-9, 1991. This is the third in a series of biannual MFDBS conferences, which in future will be held together with ICDT, the International Conference on Database Theory. MFDBS 91 covers new developments in theoretical aspects of database and knowledge base systems and the design of databases and knowledge bases. Topics of the conference are: database and knowledge base models, deductive database and knowledge base systems, logical, algebraic and combinatorial fundamentals of database theory and design of databases, object-oriented databases and object-oriented modeling, fundamentals of query languages, transaction processing, distributed databases, concurrency control, access strategies, recovery, security, privacy, safety, fundamentals for integrity constraints and consistency in databases, models for database machines, models for user interfaces, design and implementation of non-standard databases.