What role should social science play in the work for transforming society towards sustainability? The background for this question is that despite massive investments in environmental research and the accumulation of data on the human impact on the environment, action remains insufficient. The severity of the current situation has led to the conclusion that moderate change is not enough; there is a need for a fundamental transformative change of society. How social science expertise should contribute to this is a fundamental epistemic and normative question and is the point of departure for this paper. This paper aims to develop a theory of social scientific environmental expertise. It first gives a broad account of expertise and its current landscape. It then develops a pluralistic approach, where expertise can take many forms, but should be reflexive, critical, and constructive. Finally, it stresses the crucial role that social science expertise has to play in the work for transformative change, not least to broaden environmental problems and their complexities, so that society is better equipped to undergo sustainable transformation.