'Grey literature' is defined 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." This is the Luxembourg definition, developed and approved during the Fourth International Conference on Grey Literature in 1999.

The value of grey literature has been formally acknowledged by the evidence-based research organisations, the Cochrane Collaboration, the Campbell Collaboration and the Joanna Briggs Institute, all of whom require their reviewers to also search the grey and unpublished resources in order to avoid publication bias.

Examples of sources of grey literature