An improved XML query system represents an XML document as a stream of discrete events with each event representing a portion of the document as the document is being parsed. Expression-based event matching such as XPath can be performed against the event stream using a stack or tree to keep only the relevant contexts such as XPath context in memory. Observers can be used to listen for matching events. Matching events can then be routed for processing by appropriate objects or components and returned to the event stream if necessary.