The chapter argues that uncertainty over both the definition and influence of think tanks has resulted in a literature with different and incompatible views on think tanks and their influence. Nevertheless, knowledge of think tanks has been advanced. At the national level studies have nuanced our understanding of think tanks by showing their historical roles and their interaction with other organizations such as the media and political parties. Comparatively, studies have looked at differences in institutional context drawing on a wide array of data to show how their roles vary and compare across countries and levels of government. Overall the literature on think tanks is rich on contextual knowledge but remains divided on how to interpret think tank power in and across political systems.