Grey literature (or gray literature; see spelling differences) are materials and research produced by organizations outside of the traditional commercial or academic publishing and distribution channels. Common grey literature publication types include reports (annual, research, technical, project, etc.), working papers, government documents, white papers and evaluations. Organizations that produce grey literature include government departments and agencies, civil society or non-governmental organisations, academic centres and departments, and private companies and consultants.
Grey literature may be made available to the public, or distributed privately within organizations or groups, and may lack a systematic means of distribution and collection. The standard of quality, review and production of grey literature can vary considerably. Grey literature may be difficult to discover, access, and evaluate, but this can be addressed through the formulation of sound search strategies.
The concept of grey literature has gradually emerged since the 1970s. When Charles P. Auger published the first edition of his landmark work on "reports literature" in 1975, he did not use the term "grey literature". Nevertheless, his account of this "vast body of documents", with its "continuing increasing quantity", the "difficulty it presents to the librarian", its ambiguity between temporary character and durability, and its growing impact on scientific research, was entirely compatible with what is now called grey literature. While acknowledging the challenges of reports literature, he recognized that it held a "number of advantages over other means of dissemination, including greater speed, greater flexibility and the opportunity to go into considerable detail if necessary". For Auger, reports were a "half-published" communication medium with a "complex interrelationship [to] scientific journals". In the second edition of his book, published in 1989, he adopted the term "grey literature".
The so-called "Luxembourg definition", discussed and approved at the Third International Conference on Grey Literature in 1997, defined grey literature as "that which is produced on all levels of government, academics, business and industry in print and electronic formats, but which is not controlled by commercial publishers". In 2004, at the Sixth Conference in New York City, a postscript was added for purposes of clarification: grey literature is "...not controlled by commercial publishers, i.e., where publishing is not the primary activity of the producing body". This definition has since been used extensively and is widely accepted.