Why might some protesters feel a stronger moral obligation to be at a demonstration than others standing beside them? Protest research has long treated moral motivation as something that drives participation rather than as a subject to be explained. This article takes up that question by examining what shapes the duty to protest – the belief that one ought to participate in protest because it is the right thing to do. Drawing on data from the Caught in the Act of Protest: Contextualising Contestation (CCC) surveys, I analyse 9,925 respondents across 67 street demonstrations in seven European countries. The results show that anger about the protest issue is the strongest individual-level predictor of the duty to protest, followed by dissatisfaction with how democracy performs. At the demonstration level, connective action events, position issues, and publicly moralised issues are each associated with a heightened sense of duty. By contrast, prior protest participation shows no independent association with the duty to protest, but its effects depend on whether demonstrations are publicly or privately moralised. At publicly moralised events, experienced and first-time participants report comparably strong levels of duty. At privately moralised events, the sense of duty is much weaker among less experienced protesters but rises with experience. These findings offer the first empirical account of variation in the duty to protest, showing that both individual characteristics and the protest settings in which protesters mobilise explain DtP.