This thesis studies how sick leave legitimacy is managed in interaction and develops an empirically driven conceptualization of ‘legitimacy work’. The thesis applies an ethnomethodological framework that draws on conversation analysis, discursive psychology, and membership categorization analysis. Naturally occurring interaction is examined in two settings: (1) multi-party meetings at the Swedish Social Insurance Agency, in which participants assess and discuss the ‘status’ of the sick leave and plan for work rehabilitation; (2) peer-based online text-in-interaction in a Swedish forum thread that gathers people on sick leave. The thesis shows how mental states, activities and alternative categories function as resources for legitimacy work. However, such invocations are no straight-forward matter, but impose additional contingencies. It is thus crucial how they are invoked. By detailed analyses of the interaction, with attention to aspects such as lexicality and delivery, the thesis identifies a range of discursive features that manage sick leave legitimacy. Deployed resources are also subtle enough to be deniable as legitimacy work, that is, they also manage the risk of an utterance being seen as invested or biased. While legitimate sick leave is a core concern for Swedish policy-making, administration, and public debate on sick leave, previous research has for the most part been explanatory in orientation, minding legitimacy rather than studying it in its own right. By providing detailed knowledge about the legitimacy work that people on long-term sick leave do as part of both institutional and mundane encounters, the thesis contributes not only new empirical knowledge, but a new kind of empirical knowledge, shedding light on how the complexities of sick leave play out in real-life situations. Traditional sociological approaches have to a significant extent treated legitimacy as an entity with beginnings and ends that in more or less direct ways relate to external norms and cognitive states, or that focus on institutions, authority or government. By contrast, the herein emerging concept ‘legitimacy work’ understands legitimacy as a locally contingent practicality – a collaborative categorially oriented accomplishment that is integral to the interactional situation.