Recent literature has pointed to out-of-school science education as an important actor in moving towards a sustainable future. However, the fuzziness and complexity of sustainability often challenges the educational efforts of out-of-school settings, such as museums, science centres, zoos, aquaria and planetariums. Denmark is a country often lauded for progress towards sustainability, providing a strong platform for researching sustainability education practice in out-of-school settings. The project is framed by the anthropological theory of didactics, which concerns itself with the diffusion of scientific knowledge through society and social institutions (e.g. museums), and tackles three research questions. First, I operationalise sustainability in ways meaningful to the educational missions of these institutions, by accounting for the features of sustainability science and policy, as well as the different specific strengths of out-of-school settings. After synthesising preliminary criteria for good practice, I subsequently illustrate these findings in the form of an institutionally specific reference model. Next, I map the landscape of sustainability education in out-of-school settings across Denmark, in terms of the on-site educational programmes with strong sustainability content offered for visiting school classes between 10-12 years old. The study finds a relative scarcity of out-of-school sustainability programmes in Denmark, with zoos and aquaria offering the majority, as well as the influential role of the formal education sector on developing these programmes. Finally, I select strong candidates for effective sustainability education based on the previous work, to describe good practice through a search for expressions of sustainability agency among children. This research culminates in the development of guidelines for sustainability education in out-of-school settings, and engages stakeholders in employing these guidelines through a Final Workshop event. I hope that the guidelines can support out-of-school settings in the difficult process of operationalising sustainability for their education practice. Ultimately, the project qualifies the practices of out-of-school settings in their endeavours to prepare us all for a sustainable future.