If you are not writing a document from scratch, then to use the SGML system you will have first to convert existing documents from their current form to SGML - in the jargon this is called upconversion. Two sets of upconverter programs exist: one for the main text of Starlink documents in their LaTeX form, and one for source code which contains prologues which are written in the normal Starlink way. In the following sections these upconverter programs are examined in turn, and then an example recipe for converting an existing SUN is given.