You've gotten some other good responses, but I thought I'd mention the LoC and OCLC sites on MARC if you haven't seen them yet. First, the LoC site at http://www.loc.gov/marc/. This is what I use as a guide and a reference. Some folks prefer the OCLC docs http://www.oclc.org/bibformats/en/, particularly if they're an OCLC member. Of course, these apply to MARC-21 and not UniMarc. Not sure what good resources are out there for UniMARC. Jon Gorman