When Dartmouth Journal Services began building ProofExpress, an online, XML-based proofing and editing system for STM journals, we knew that the most difficult challenge would be creating an accurate change-tracking mechanism. Change tracking is an essential feature, both to ensure that author corrections conform to journal style and to catch any changes to data or claims. The system must not only track each insertion, deletion, and formatting change, it must also give production editors the ability to accept or reject changes without breaking the XML. ProofExpress is built on SDL LiveContent Create (formerly Xopus). We use its extensive API to add custom elements and attributes to mark changes in the XML. The XML is then transformed through XSLT to group and nest changes so that they can be acted upon by the production editor. To prevent breaking the XML during this process, a rule engine enforces the order of acceptance and rejection of changes.