Ageing is associated both with frailty and cognitive decline. The quest for a unifying approach has led to a new concept: cognitive frailty. This systematic review explores the contribution of cognitive assessment in frailty operationalization. PubMed, Web of Knowledge and PsycINFO were searched until December 2016 using the keywords aged; frail elderly; aged, 80 and over; frailty; diagnosis; risk assessment and classification, yielding 2863 hits. Seventy-nine articles were included, describing 94 frailty instruments. Two instruments were not sufficiently specified and excluded. 46% of the identified frailty instruments included cognition. Of these, 85% were published after 2010, with a significant difference for publication date (X 2 = 8.45, p <.05), indicating increasing awareness of the contribution of cognitive deficits to functional decline. This review identified 7 methods of cognitive assessment: dementia as co-morbidity; objective cognitive-screening instruments; self-reported; specific signs and symptoms; delirium/clouding of consciousness; non-specific cognitive terms and mixed assessments. Although cognitive assessment has been increasingly integrated in recently published frailty instruments, this has been heterogeneously operationalized. Once the domains most strongly linked to functional decline will have been identified and operationalized, this will be the groundwork for the identification of reversible components, and for the development of preventive interventional strategies.