This paper describes the active object-oriented database system ACOOD developed at the universities of Skövde and Exeter. ACOOD adds active functionality on top of the commercially available Ontos DB. The active behaviour is modelled by using Event-Condition-Action (ECA) rules. ACOOD offers all essential functionality associated with an active database. The semantics and user interface have been clearly defined in order to produce a prototype that can be used to develop database applications. The historical background of active databases and the development of ACOOD are covered in the paper together with a detailed description of the latest, redesigned version of the system. There is also a discussion of experience gained through the work with ACOOD and a comparison with similar systems.