This paper examines an emerging technology known as active databases. The focus is on issues which need to be considered in their development and the way they have been addressed in currently implemented systems. We examine what defines active behavior, why it is useful, the semantics of its specification and choices that have to be made when realising it in an actual system. We see that a multitude of options exist when designing an active database and future research will prove to be both necessary and fruitful. 1 Introduction Traditional database systems provide a mechanism for storing large amounts of data and an interface for manipulating and querying this data. They are, however, passive in the sense that their state can only change as a result of outside influences. In contrast, an active database is a system providing the functionality of a traditional database and additionally is capable of reacting automatically to state changes, both internal and external, without user inte...