Methods for reviewing research, such as systematic reviews and syntheses, are becoming increasingly common in conservation. It is widely recognised that grey literature, research not published in traditional academic journals, forms a vital part of the evidence base of these reviews. To date guidance and practice in searching for and including grey literature in conservation reviews has taken a broad approach, involving searching of a wide variety of resources. We argue that there are two distinct forms of grey literature and that each must be considered separately in order to assess potential importance and an appropriate searching strategy for every review undertaken. ‘File drawer’ research is as yet unpublished academic research that is important for countering possible publication bias and can be targeted via specific repositories for preprints, theses and funding registries, for example. ‘Practitioner-generated research’ includes organisational reports, government papers and monitoring and evaluation reports, and is important for ensuring comprehensiveness in conservation reviews. By considering the relative importance and appropriate strategies for inclusion of both types of grey literature, reviewers can optimise resource efficiency and comprehensiveness, and minimise publication bias.