||Document engineering is the systematic development of document presentations, representations and document tools. We particularly address document engineering for the Extensible Markup Language (XML); both its Document Type Definitions (DTDs) and its DTD-conforming documents. Our thesis is that document engineering should be firmly based on explicit, formal document models. This precept is well accepted and practiced in, for example, the database community but it is, apparently, less accepted in the XML community. We want to change this position. As a first step in the demonstration of the value of explicit, formal document models we derive and discuss an explicit, formal model for XML DTDs and their DTD-conforming documents. This new model captures what can be considered to be the essence of an XML DTD and its DTD-conforming documents.