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Background: This thesis explores the concept of frailty, as a latent vulnerability in older people, with the aim of refining its measurement by generating a new measure of frailty - the British Frailty Index (FI). This index was developed and validated in a cohort of community-dwelling older women, the British Women's Heart and Health Study (BWHHS), in 23 towns in Britain. Findings were replicated in another large Medical Research Council (MRC) Assessment of Older People study Methods: A systematic literature review examined the evolution of the concept and definitions of frailty. A meta-analysis on the prognostic value of current frailty measures confirmed extensive heterogeneity in the prediction of all-cause mortality despite consideration of age, sex, type of measure and duration of follow up. A 'General Specific' model of frailty was derived from factor analysis in the BWHHS population and replicated in the MRC cohort. Construct, external criterion and predictive validity of the British FI were assessed and its performance compared to another widely used index - the Canadian Frailty Index - with single indicators of frailty. Results: Frailty was explained by seven factors; physical ability, cardiovascular and respiratory disease and symptoms, visual impairments, other comorbidities, psychological problems and physiological measures. Associations with frailty included increased age, female sex, smoking, living alone, not living in own home, poor social contact and low socioeconomic position. Frailty was an independent predictor of all-cause mortality in both cohorts and predicted hospitalization and institutionalization in the MRC study, performing better than the Canadian Index. Conclusion: This thesis provides better understanding of the multi-dimensional domains of frailty in older people. The British FI demonstrates validity in relation to adverse outcomes, provides a more reliable measurement tool and its application offers further opportunities for the prevention, detection and treatment of frailty at a clinical level.