Frailty is most often conceptualized as an age-related medical syndrome with a biological basis that results in substantial vulnerability stressors and adverse health outcomes. It is increasingly thought of as the geriatric syndrome that underlies all other geriatric syndromes. Multiple measurement tools have been developed that have helped to differentiate frail from more robust or resilient older adults; and these tools enabling the characterization of and consequences of frailty, as well as its underlying biology. The recognition of frailty as a condition of advanced vulnerability has led to the development of interventions and novel approaches to integrate frailty into clinical practice. The following chapter describes the conceptualization of frailty, surveys the vast array of tools used to identify frailty, provides demographic information and epidemiological characterizations of causes and consequences of frailty, identifies important biological and environmental drivers of frailty, and describes the clinical efforts underway to better utilize frailty tools.