The need to preserve scientific, scholarly and cultural data has long been recognized. These data sets are valuable and many of them are either impossible to reproduce (e.g. climate and demographic data) or can only be recovered at enormous costs (e.g. data from high energy physics experiments). While substantial investment has been made in archiving and preserving conventional forms of these objects, such as documents, images and numerical data in some file format, the need to preserve entire databases has only recently emerged. Databases differ from fixed digital objects studied in the past, in that they change over time, they have internal structure, and they include schemas and integrity constraints, which are basic for the current and future interpretation of the data. Increasingly, database technology is being used in the storage of large numerical scientific data sets.