XML is a now a dominant standard for storing and exchanging information.  With
its increasing use in areas such as data warehousing and e-commerce, there is
a rapidly growing need for rule-based technology to support reactive
functionality on XML repositories.  Event-condition-action (ECA) rules
automatically perform actions in response to events and are a natural facility
to support such functionality.  In this paper, we study ECA rules in the
context of XML data.  We define a simple language for specifying ECA rules on
XML repositories. The language is illustrated by means of some examples, and
its syntax and semantics are then specified more formally.  We then
investigate methods for analysing and optimising these ECA rules, a task which
has added complexity in this XML setting compared with conventional active
databases.