XMLTK: An XML toolkit for scalable XML stream processing. We describe a toolkit for highly scalable XML data processing, consisting of two components. The first is a collection of stand-alone XML tools, s.a. sort- ing, aggregation, nesting, and unnesting, that can be chained to express more complex restructurings. The second is a highly scalable XPath processor for XML streams that can be used to develop scalable solutions for XML stream applications. In this paper we dis- cuss the tools, and some of the techniques we used to achieve high scalability. The toolkit is freely available as an open-source project.