This book analyses situations in which individual agents, who might be different from each other, interact and produce behaviour on the aggregate level which does not correspond to that of the average actor. This leads to aggregate outcomes which would be impossible to explain in a more standard approach. Aggregation generates structure and, as a result, interaction and heterogeneity can be handled and we no longer have to rely on the over-simplified reduction of the behaviour of the economy to that of a "rational" individual.