There have been increasing demands for a system to sup- port DTD -conforming XML to XML transformations: given any target DTD D and a source XML document S, extract data from S and construct a target XML document T such that T conforms to the predefined D. The need for this isevident in, e.g., data exchange, security views, and data inte-gration. Popular XML query languages (e.g., XQuery, XSLT ) cannot guarantee DTD conformance in XML to XML trans- formations. Type inference and (static) checking are tooexpensive to be used in practice; worse still, they provide noguidance for how to ensure DTD -conformance. In response to the need we propose TREX (TRansfor- mation Engine for XML ), a middleware system for DTD - conforming XML to XML transformations. TREX is based on the novel notion of XTG ( XML Transformation Gram- mar), which extends a DTD by incorporating XML queries into element type definitions. This allows one to specify howto extract relevant data from a source XML document via the queries, and to construct a target XML document directed by the embedded DTD . TREX efficiently evaluates XTG s by implementing several optimization techniques. XTG s and TREX provide the first systematic method and practical sys- tem to support DTD -conforming XML transformations.