Frailty is a term used as a marker of vulnerability, identifying individuals with a diminished capacity to respond to external stressors. Those who are frail are at increased risk of death, institutionalisation and worsening disability. While the associations of frailty with increasing chronological age, female gender, functional dependence and chronic disease are now well described, the aetiology of frailty remains less clearly understood. The growing body of evidence linking inflammation and frailty in older people, an association that seems consistent across different frailty definitions, is therefore of great interest to research gerontologists. However, further studies are needed to establish the exact nature of this relationship. Inflammation may be primarily causal, a compensatory response to viral antigens or subclinical disease or an epi-phenomenon, merely a marker of another key pathophysiological process such as excessive oxidative stress. Furthermore, it has recently been recognised that frailty is most strongly associated with a combination of immunological and physiological impairments rather than a single biomarker. This supports the conceptualisation of ageing as the progressive accumulation of damage to a complex system resulting in aggregate loss of system redundancy. The frail older person is analogous to a complex system without redundancy: capacity to respond to external stressors is reduced. A critical mass of abnormalities across different systems may therefore be a more important determinant of frailty than any individual pathway.