What is environmental expertise? The background to this question is that many scholars consider environmental expertise crucial for discovering, diagnosing, and solving environmental problems but do not discuss in any depth what constitutes expertise. By investigating the meaning and use of the concept of expertise in three general theories within environmental sociology—the treadmill of production, risk society, and ecological modernization—and findings from science and technology studies (STS), this article develops a sociological understanding of environmental expertise: what it is and how it is acquired. Environmental expertise is namely about group belonging and professional socialization around specialized skills; that is, it concerns both substantial competence and social recognition. The implications of this general view on expertise are then used to enrich theories in environmental sociology.