Increasingly in economics what had been considered to be unusual and unacceptable has come to be considered usual and acceptable, if not necessarily desirable. Whereas it had been widely believed that economic reality could be reasonably described by sets of pairs of linear supply and demand curves intersecting in single equilibrium points to which markets easily and automatically moved, now it is understood that many markets and situations do not behave so well. Economic reality is rife with nonlinearity, discontinuity, and (...) a variety of phenomena that are not so easily predicted or understood. At the same time the broad coherence of economic systems is more impressive than ever in the face of such phenomena. The order of the economy appears to emerge from the complex interactions that constitute the evolutionary process of the economy. These phenomena have come to be labeled as complexity in economics. Even what seems simple in economics generally arises from behavior not reflecting rational expectations; we live in a world that reflects the enormous variety and diversity of humanity in their knowledge, attitudes, and behaviors, interacting with each other in an enormous range of institutional frameworks. What emerges in the aggregate may have little to do with what happens at the individual level. But this aggregate cannot be simply described by some set of aggregate equations. It emerges out of the soup of the individual and particular with all its multiform interactions and peculiarities. (shrink)