<p>Fragility is a term widely used in research and policy, yet its conceptualization remains contested even within fields. We develop a general framework for understanding and measuring fragility. It applies across fields and is oriented toward policy use. The framework provides practitioners with a common structure for thinking about fragility, a set of candidate measures, and guidance for constructing them. Fragility is defined as the ease with which an object fails. We distinguish it from risk, resilience, and vulnerability and identify five types of measures: critical stress, failure condition, conditional probability, unconditional probability, and composite indices. We examine the use of these measures and identify the contexts in which they are most appropriate. Together, they imply a trade-off between causal interpretability—the ability to explain how failure occurs—and operational feasibility—the ease with which a measure can be constructed with reasonable validity. We then discuss the use of fragility measures in policy and develop a protocol for constructing fragility measures, which we apply to a case study of urban areas exposed to wildfire. Even small differences in failure definitions lead to different fragility measures and policy implications.</p>