Environmental expertise is a frequently used concept in environmental discussions and in environmental research. It is often seen as crucial for environmental policy-making and environmental discourses. Through its competence, authority and mandate, expertise offers guidance to organizations and individuals concerning how to understand, address and manage environmental issues. At the same time, expertise is often questioned; its capacity to deliver both trustworthy and relevant knowledge is contested. This entry discusses the meaning of expertise, both the social and epistemic base for becoming an expert and the importance it has. Environmental expertise is a key concept for environmental sociology but, at the same time, is a concept that needs to be further elaborated and studied.