Grey literature (or gray literature) is 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 organizations, academic centres and departments, and private companies and consultants.

Grey literature may be difficult to discover, access, and evaluate, but this can be addressed through the formulation of sound search strategies. 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.