Gray or grey literature has long been considered the proverbial needle in the haystack. It is commonly defined as any documentary material that is not commercially published and is typically composed of technical reports, working papers, business documents, and conference proceedings. The greatest challenges involved with these items are the process of identification, since there is limited indexing, and acquisition, since availability is usually marred with uncertainty. Added to this is the absence of editorial control, raising questions about authenticity and reliability. Yet despite these considerations, gray literature is continually referenced in scholarly articles and dissertations and therefore remains an issue that academic librarians must contend with.
-- Gray literature: Resources for locating unpublished research, Brian Matthews, C&RL News, March 2004; Vol. 65, No. 3