This thesis investigates why and how municipalities organize to address grand challenges. Previous research shows that municipalities have increased their policymaking and organizing in relation to grand challenges, often without any national regulations forcing them to do so. The rationales, processes, and mechanisms underpinning this type of municipal voluntarism are understudied. The research is based on the case of Scandinavian municipalities and their efforts to prevent terrorism, extremism, and radicalization (TER). From playing a miniscule role in Scandinavian counter-terrorism policies until the 2010s, municipal employees such as teachers, social workers, and youth workers have in current practice become the backbone of the fight against TER. Municipalities generally have little or no strategic or practical experience of preventing TER, resulting in extensive uncertainty and ambiguity as to how to organize the relevant efforts. In this thesis, the process leading from grand challenge to municipal organizing is framed as a translation process. The analysis uses concepts from sociological institutional theory and social movement studies, and is informed by data from newspaper articles, municipal policies, interviews, and observations. The findings are presented in four papers. This thesis shows how the decentralization of a grand challenge from being an international or national to a municipal responsibility is a multi-layered, highly discursive translation process that is dependent on reframing a challenge as a local one. Regarding TER, the local frame was based on a new institutional vocabulary, triggered and legitimized by critical events, which elite actors used to localize the grand challenge. Once localized, institutional pressure was exerted on municipalities to organize preventive efforts. While institutional pressure caused rapid organizational activity, it also led to the ambiguous translation and editing of concepts and preventive approaches with unintended, paradoxical, and problematic consequences. Many of the observed organizing activities centered on rhetorical efforts to legitimize the challenge and its associated concepts and practices. This was a consequence of the grand challenge being contested locally, since it introduced a new institutional logic that conflicted with those dominating the local institutional context.