In this article we examine aspects of the different arguments for the environmental advantages of co-housing compared with individual households. The analysis is structured around four main questions, which are argued to be decisive for the question of co-housing and sustainability. The first is whether co-housing offers better opportunities for choosing and using more sustainable technologies, which also relates to the question of whether co-housing offers better opportunities for building smaller and denser and thus more energy efficient buildings. The second and third questions are socially oriented; one relates to the claim that co-housing can support pro-environmental behaviour among residents as they can support each other's norms and practices. The fourth and last claim relates to a discussion of co-housing as a more sustainable opportunity especially for people living alone, as the growing number of small households is an emerging sustainability problem. The empirical analyses are based on the results from a Danish study of eco-villages including a survey, interviews with representatives of the eco-village movement and a detailed case study of a group of people in the process of establishing a new cluster in an existing eco-village. The aim of the article is to contribute to the general discussions about co-housing and sustainability. The study adds nuances to this discussion and shows that the answer is not as straightforward as presented in much of the literature.