This paper describes the design, implementation, and performance characteristics of a commercial XQuery processing engine, the BEA streaming XQuery processor. This XQuery engine was designed to provide high performance for message-processing applications, i.e., for transforming XML data streams. The engine is a central component of the 8.1 release of BEA’s WebLogic Integration (WLI) product. The BEA XQuery engine is fully compliant with the August 2002 draft of the W3C XML Query Language specification and we are currently porting it to the latest version of the XQuery language (July 2004). A goal of this paper is to describe how a fully compliant yet efficient XQuery engine has been built from a few relatively simple components and well-understood technologies.