Yes, grey literature can be included in a systematic review.  And in fact, this is a best practice in order to reduce publication bias, access more international literature, and in some cases, to obtain more current, relevant data.  For more information on grey literature, check out our Grey Literature in Health Sciences LibGuide.

Clinical trial registries are just one example of grey literature and can be tremendously valuable for systematic reviews.  Some examples of clinical trial registries include:  ClinicalTrials.gov, metaRegister of Controlled Trials, WHO International Clinical Trials Registry, EU Clinical Trials Register, HSRProj, and the ISRCTN Register.