Environmentally sustainable construction is now recognised as a significant element of the broader sustainable development agenda and planners are being called upon to play a role in delivering more sustainable patterns of construction and development. This puts particular demands on the knowledge resources of planners since knowledge is implicated in the power relations between planners and developers. This paper examines the interrelationship between knowledge of environmentally sustainable construction and practice in planning departments. Drawing on a survey of and interviews with planners in London, it discusses the construction of knowledge within the dynamics of planning organisations and the potential for learning to promote a more sustainable built environment. Wenger's concept of communities of practice frames the analysis, alongside consideration of the translation of knowledge into bureaucratic and usable forms and the role of knowledge brokers in this process.