Electronic publications are not inseparably tied to a particular physical format, but can be copied to new storage media with little or no loss. However, experience about the long-time durability of the media is not yet available. An even greater danger than deterioration of the storage media lies in the technological obsolescence of the reading devices which are necessary to use electronic publications. Technology is changing rapidly; hence, in the digital environment ``preservation means copying, not physical preservation'' (Graham, 1994). This process usually is called technology refreshing; in an even broader concept, the Task Force on Archiving of Digital Information refers to it as ``migration'' which is defined as ``a set of organized tasks to achieve the periodic transfer from one hardware/software configuration to another or from one generation of computer technology to a subsequent one'' (Garrett and Waters, 1996). The purpose of migration is to retain the ability for clients to use digital objects in the face of constantly changing technology. Copying should take place at regular intervals in order to avoid unexpected and possibly unbridgeable gaps in the availability of necessary hardware and software. Information should be encoded in a system-independent format, for instance SGML, as the ability to migrate and refresh will be much greater for such a richly tagged format than it is for a page image format like PDF or Postscript.