Agglomeration economics explores the role of economists in the public policies of the cities in the UK since the 1970s. The main objective is to describe how the idea of agglomeration emerged from the work of economists and economic geographers more generally, and how this body of knowledge became influential in policy thinking. The case study is based on an intellectual history of the idea that “agglomeration is good for you” as well as an analysis of the rise of economics’ tools within policy discourses regarding cities’ planning and management.