“Follow the money” is now a well-established practice of policing and intelligence. The fight against money laundering and terrorist financing is located at the interface of finance and security. Financial surveillance is based on cooperation between two groups of actors who have different motivations and different know-how. Indeed, the blurred notion of “dirty money” has become the meeting point of a constellation of interests and of two logics of action that are related to law enforcement authorities and financial institutions. With respect to the anti-money laundering apparatus in United Kingdom, the article highlights the articulation of these logics of action around a misunderstanding regarding the meaning of the fight against “dirty money”. Here, the misunderstanding does not represent an obstacle to the elaboration and the operationalization of a specific policy. On the contrary, it is a precondition for this policy. While there are oversights and differences of interpretation about the “problem” to be addressed and the aims to be achieved, this situation paradoxically contributes to the formation of the mixed battalion of financial surveillance professionals.