Abstract. Anomaly detection systems assume that a certain deviation from the regular behaviour of a system can be an indicator for a security violation. They proved their usefulness to networks and operating systems for a long time, but are much less prominent in the field of databases. Relational databases operate on attributes within relations, ie, on data with a very uniform structure, which makes them a prime target for anomaly detection systems. This work presents such a system for the database extension and the user interaction with a DBMS; it also proposes a misuse detection system for the database scheme. In a comprehensive investigation we compare two approaches to deal with the database extension, one based on reference values and one based on ∆-relations, and show that already standard statistical functions yield good detection results. We then apply our methods to the user interaction, which is split into user input and DBMS behaviour. All methods have been implemented in a semi-automatic anomaly detection tool for the MS SQL Server 2000.