Databaseresearchisa?eldofcomputersciencewheretheorymeetsapplications. Many concepts and methods, that were regarded as issues of theoretical interest when initially proposed, are now included in implemented database systems and related products. Examples abound in the ?elds of database design, query languages, query optimization, concurrency control, statistical databases, and many others. The papers contained in this volume were presented at ICDT 99, the 7th - ternationalConferenceonDatabaseTheory, inJerusalem, Israel, January10 12, 1999. ICDT is an international forum for research on the principles of database systems. It is a biennial conference, and has a tradition of being held in beau- ful European sites: Rome in 1986, Bruges in 1988, Paris in 1990, Berlin in 1992, Prague in 1995, and Delphi in 1997. From 1992, ICDT has been merged with another series of conferences on theoretical aspects of database systems, The Symposium on Mathematical Fundamentals of Database Systems (MFDBS), that was initiated in Dresden (1987), and continued in Visegrad (1989) and Rostock (1991). ICDT aims to enhance the exchange of ideas and cooperation in database research both within uni?ed Europe, and between Europe and the other continents. ICDT 99 was organized in cooperation with: ACM Special Interest Group on Management of Data (Sigmod) IEEE Israel Chapter ILA The Israel Association for Information Processing EDBT Foundation ICDT 99 was sponsored by: The Hebrew University of Jerusalem Tel Aviv University Tandem Labs Israel, a Compaq Company This volume contains 26 technical papers selected from 89 submissions."

The Fourth International Workshop on Database Programming Languages - Object Models and Languages (DBPL-4) took place in Manhattan, New York City, 30 August-1 September 1993. The areas of interest and the format of DBPL-4 focused on the integration of programming languages, object models, type systems and database systems. As in the previous DBPL workshops, the setting was informal, allowing the participants to actively discuss and argue about the ideas presented in the talks. The comments and remarks made by the participants during and after the presentations were taken into account in the preparation of the final versions of the papers. The result, we believe, is a set of excellent papers. The DBPL sequence is closely related to the sequence of International Workshops on Persistent Object Systems (POS), first started in 1985. While the DBPL workshops focus on language and model issues, the POS workshops have focused on implementation issues; thus the two sequences complement each other. Many researchers participate in both workshop series. The eight sessions of the technical program of DBPL-4 were as follows: 1. Bulk types and their query languages (two sessions). 2. Object models and languages. 3. Data types with order. 4. Mechanisms to support persistence, reflection, and extensibility. 5. Query optimization and integrity constraints. 6. Logic-based models. 7. Implementation and performance issues.