Making the transition to a green economy is a major policy driver in the UK and other countries. Entrepreneurs are suggested as being at the forefront of this transition and as a driving force for sustainability. These “green entrepreneurs” may represent a new type of entrepreneurial behaviour combining economic, environmental and social aims. In this paper, we present empirical work conducted with green entrepreneurs in the UK green building sector. Buildings have significant impacts on the environment, both in terms of materials and post-construction energy demands. Drawing on sustainability transitions theory, we examine the role of green entrepreneurs in affecting change and suggest that green building niches are less consensual than previously theorised. In theorising green entrepreneurs, we also point to the need to consider them within wider networks of activity rather than as lone actors and the implications this has for policy