We introduce a new formal semantics for active databases that relies on a transaction rewriting technique. A user defined transaction, which is viewed here as a sequence of atomic database updates forming a semantic unit, is translated by means of active rules into induced one(s). Those transactions embody active rule semantics which can be either immediate or deferred. Rule semantics, confluence, equivalence and optimization are then formally investigated and characterized in a solid framework that naturally extends a known model for relational database transactions. Keywords: Active databases, rule semantics, transaction equivalence, confluence, optimization. 2 1 Introduction Active databases are based on rules that allow us to specify actions to be taken by the system automatically, when certain events occur and some conditions are met. It is widely recognized that these active rules provide a powerful mechanism for the management of several important database activities (e.g., c..