Contemporary rising powers have often pursued a hesitant and ambiguous foreign-policy and have belied the expectations of potential followers and established powers who would want them to engage more actively in global and regional governance. The existing analytical toolbox of International Relations does not offer suitable concepts to make sense of the widespread phenomenon of states that pursue hesitant, inconsistent courses of action and do not bring to bear their power resources to coherently manage international crises that potentially affect them. A notion that is frequently employed to describe this peculiar type of foreign policy is that of ‘reluctance’, but this concept has not been systematically defined, discussed or theorized. This article aims to introduce the concept of reluctance into the field of International Relations. It develops a conceptualization of reluctance by identifying the concept’s semantic field and discussing how reluctance relates to the similar but distinct notions of exceptionalism, isolationism, under-aggression and under-balancing (concept reconstruction); on that basis, the article outlines the constitutive dimensions of reluctance — hesitation and recalcitrance — and their operationalization (concept building). Several illustrative cases of (non-)reluctant rising powers are used to exemplify the concept structure and to show the analytical usefulness of the concept of reluctance, which refers to a distinct set of phenomena that are not addressed by other concepts in International Relations. An application of the concept allows us to identify policy shifts and differences across issue areas, as well as open up avenues for further research.