The steady ageing of the population poses growing challenges for welfare systems, given the rising number of older individuals living with compromised health. In this context, the concept of “frailty” plays a crucial role: being able to identify frail individuals means detecting early signs of vulnerability and guiding targeted preventive interventions. This volume explores the theoretical and methodological foundations of the various perspectives on frailty, offering a comprehensive and structured overview of the tools currently available for its measurement. Readers are guided through a structured journey that encompasses key definitions, measurement tools, and methodological approaches. Each element is examined with regard to its strengths, limitations, and empirical applicability, through a statistical and methodological lens and with careful attention to data sources. In addition to physical frailty, significant attention is given to cognitive and social frailty – dimensions often overlooked but essential to an integrated and multidimensional understanding of the phenomenon. This resource is designed for healthcare professionals, social workers, statisticians, researchers, and policy makers who aim to study, measure, and address frailty through a rigorous, quantitative approach.