Abstract: A document assembly or document automation system includes an assembler for generating an instance document on the basis of a source document and one or more logic source documents referenced by the source document. The source document and logic source documents are XML documents including at least one XML processing instruction. The source document and logic source documents are valid with respect to XML schema. The system generates an instance document in HTML, PDF or RTF format by resolving variables in the source document and/or logic sources using one or more data sources. This may involve performing one or more interview rounds with a user of the system, access to a database, and/or evaluation of a function defined in one of the documents. The system includes an editor for creating and maintaining source documents and logic source documents whilst maintaining their validity with respect to the appropriate XML schema.