This chapter builds on original fieldwork conducted while involved in a European think and do tank called Epode European Network (EEN), whose goal is to disseminate ‘best practices’ regarding health promotion and childhood obesity prevention. Resorting to ‘participant observation,’ the author gained access to numerous work meetings, symposia and congresses. He also produced document analyses and over 30 semi-structured interviews with relevant stakeholders in Europe. Based on the micro-sociological approach of ethnopraxis (Wacquant 2005), this chapter offers an insider’s point of view into the interstitial field of think tanks. Funded by agri-food corporations and the European Commission, the EEN illustrates how different social worlds (politics, academia, business, and media) are connected and attended by brokers of philanthropy specialized in corporate engagement (in-house lobbyists, public affairs professionals, head of food quality or nutrition, head of external communication, secretaries of foundations, etc.). In this context, various interdependent social games were observed that shed light on otherwise invisible ‘collusive transactions’ (Dobry 1986) where each group of actors finds specific interests in the exchanges between fields. In entering the black box of think tanks, the author relies on first-hand experience producing knowledge and recommendations on public-private partnerships to reflect on the constraints inherent to the action-oriented epistemology that underlies the crafting of policy expertise.