This paper has two main goals. The first is to study the links between the “new” economic theories, this is, the “new” trade theory, the “new” growth theory and the “new” economic geography. These are three apparently distinct strands of economics, yet they have a common motivation: the role of increasing returns and the consequent market structure (imperfect/monopolistic competition). The second goal is to present the “new” economic theories as case studies in what concerns the debate over modelling and its role in the progress of economics. Since these theories contribute fundamentally by applying new modelling techniques to old real world problems, they add something to economic knowledge to the extent that we accept formalisation as a source of progress in economics.