Grey literature is generally considered to be material published or made available by organisations or individuals, not through commercial publishers. This can include: clinical study reports, conference abstracts, government documents, organisational reports, personal communication, pre-prints, regulatory documents, study registrations, social media posts and theses.
Searching for grey literature can be problematic, as it isn't collected, organised or stored in a consistent way. You will need to be flexible in your approach, depending on the type of grey literature you need – this will differ from review to review.
Reporting your grey literature search is not as straightforward as reporting a search on a bibliographic database. You should aim to record the following information: