Recently, active behavior has received attention in the XML field to automatically react to occurred events. Aside from proprietary approaches for enriching XML with active behavior, the W3C standardized the Document Object Model (DOM) Event Module for the detection of events in XML documents. When using any of these approaches, however, it is often impossible to decide which event to react upon because not a single event but a combination of multiple events, i.e., a composite event determines a situation to react upon. The paper presents the first approach for detecting composite events in XML documents by addressing the peculiarities of XML events which are caused by their hierarchical order in addition to their temporal order. It also provides for the detection of satisfied multiplicity constraints defined by XML schemas. Thereby the approach enables applications operating on XML documents to react to composite events which have richer semantics. Categories and Subject Descriptors ...