All economists are familiar with the division of the subject into microeconomics and macroeconomics. However, few economists are able to give an accurate account of what distinguishes micro and macro. The increasing interest in the microfoundations of macroeconomics' has typically attracted those who feel that economics is about the rational behaviour of individuals and who regard macro propositions as a consequence of the intentions of individuals. As a result, microfoundations' have come to be synonymous with theories of individual behaviour. However, Maarten Janssen argues that it is microeconomics' concern with the functioning of markets that underlies theories of microfoundations. This claim is substantiated by an analysis of the aggregation problem, of the foundations of equilibrium theories, of the rational expectations hypothesis, and of a model from the New Keynesian literature.