Grey literature refers to resources not available via mainstream publishing channels.
Sources of grey literature don't have publishing as their core business, but rather produce research and other documents as part of their work.
Examples
Grey literature can be tricky to find. Here are a few approaches.
Watch this video (1 minute) for instructions on grey literature searching using google advanced
Each module includes instruction and an interactive exercise to explore an issue where grey literature plays a relevant role.
Learn how to effectively find
Government reports - Clinical Trials - Statistics - Theses