This paper locates the notion of Sustainable Diet as the latest iteration in the long-term policy question: What is a ‘Good Diet’? It reviews why there is tension over broadening the notion of a Good Diet to include the environment. It contrasts simple and complex approaches to sustainability for food systems and diet in particular. It proposes a multi-criteria approach to dietary analysis and to policy guidelines. The experience of a number of countries are summarised and analysed for sources of resistance and difficulty. It proposes that the socio-cultural dimension of Sustainable Diet requires further analysis but already offers promising avenues for change.