Incidence rates of cognitive impairment and dementia are rising with the ageing population. Meanwhile, the limited success of current treatments has led to a search for early markers of dementia which could predict future progression or improve quality of life for those already suffering from the disease. One focus has been on the correlation between physical and cognitive measures with an increasing interest in the association between frailty and cognitive decline. Frailty is an age-related syndrome described as the decreased ability of an organism to respond to stressors. A number of epidemiological studies have reported that frailty increases the risk of future cognitive decline and that cognitive impairment increases the risk of frailty suggesting that cognition and frailty interact within a cycle of decline associated with ageing. This paper reviews the evidence for an association between frailty and cognitive impairment and outlines some of the mechanisms that potentially underpin this relationship from brain neuropathology and hormonal dysregulation to cardiovascular risk and psychological factors.