The chapter will introduce a working definition of the term ‘think tank’ and will give an overview over different types of and strategies employed by think tanks. In the following, a model for distinguishing distinct political and cultural environments in which think tanks operate will be introduced. Drawing on John Campbell and Ove Pedersens concept of “knowledge regimes” allows to understand the respective ecosystems of US American and German think tanks better and to formulate hypotheses about their respective position in political networks of advice giving and advocacy. The chapter lays the conceptual foundation for subsequent empirical case studies and is hence a s suitable starting position for exploring the changing roles of think tanks in disseminating climate sceptic narratives and their role in forging new political alliances.