The concept of fragility has gained an increasing relevance in development discourse. Nevertheless, it remains a fuzzy and elusive concept. This article presents a review of the literature, and identifies two main sets of definitions of fragility, which substantially differ in their focus and breadth, and that reflect the evolution of the discourse around this developmental concept. The limited consensus that is found in the literature suggests that the analytical salience and the direct operational value-added of the concept remain unclear. Nevertheless, the debate around state fragility has played an important advocacy role, and has offered key methodological insights with respect to the challenges that donors face, and what can be realistically achieved through external engagement.