This chapter elaborates a sociological understanding of environmental expertise. The background is that expertise is a frequently used concept in environmental discussions as well as in environmental research but is rarely defined in any deeper sense. By drawing on findings from Science and Technology Studies (STS), this chapter develops an understanding of expertise that is both about having skills and competence and having a social position within a specific domain, i.e. being about epistemic as well as social dimensions. This means that there is no expertise per se but different practices of expertise. Thus, the importance of expertise should not be taken for granted but rather investigated. This understanding is illustrated by examples from the climate change issue.