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Grey Literature (GL) is any materials produced by a body whose main purpose is not publishing. The major components GL are research reports, technical papers, pre-prints, and unpublished conference proceedings. Other types of materials include annual reports, data sets, unreviewed essays, policy papers, consultation papers, etc.
Because of its currency and focus, GL can be highly valuable. Because of the difficulty of collecting it, libraries generally do not offer good access to it.
Science and Medicine appear to have the most highly developed structures for collecting and offering access to GL.
The term GL is derived from German usage, and equivalent terms are in use in all major European languages. In the US GL is sometimes spelled Gray Literature. The term is a successor to the older "reports literature."
Resources: articles on GL appear regularly in a variety of LIS journals that are indexed in the usual sources. The Grey Journal, a European periodical, is the only dedicated source. There is an annual international conference on GL sponsored by GreyNet.