policy and practice information. Professionals insist on its importance for two main reasons: research results are often more detailed in reports, doctoral theses and conference proceedings than in journals, and they are distributed in these forms up to 12 or even 18 months before being published elsewhere. Some results simply are not published anywhere else. In particular, public administrations and public and industrial research laboratories produce a great deal of "grey" material, often for internal and in some cases "restricted" dissemination. The notion of evidence-based policy has also seen some recognition of the importance of grey literature as part of the evidence base; however, the term is not yet widely used in public policy and the social sciences more broadly. Problems For a number of reasons, discovery, access, evaluation and curation of grey literature pose a number of difficulties. Generally, grey literature lacks any strict or meaningful bibliographic control. Basic information such as authors, publication dates and publishing or corporate bodies may not be easily identified. Similarly, the nonprofessional layouts and formats, low print runs and non-conventional channels of distribution make the organized collection of grey literature a challenge compared to journals and books. Although grey literature is often discussed with reference to scientific research, it is by no means restricted to any one field: outside the hard sciences, it presents significant challenges in archaeology where site surveys and excavation reports, containing unique data, have frequently been produced and circulated in informal "grey" formats. Some of the problems of accessing grey literature have decreased since the late 1990s as government, professional, business and university bodies have increasingly published their reports and other official or review documents online. The informal nature of grey literature has meant that it has become more numerous as the technology that allows people to create documentation has improved. Less expensive and more sophisticated printers increased the ease of creating grey literature. And the ability to post documents on the internet has resulted in a tremendous boom. The impact of this trend has been greatly boosted since the early 2000s, as the growth of major search engines has made retrieving grey literature simultaneously easier and more cluttered. Grey reports are thus far more easily found online than they were, often at no cost to access. Most users of reports and other grey documents have migrated to using online copies, and efforts by libraries to collect hard-copy versions