The chapter deals with the complexity approach to economics, which emerged in the 1980s through the work of a group of scientists at the Santa Fe Institute for the Study of Complex Systems and at the University of Michigan. Firstly, the chapter analyzes the research program in complexity economics developed at the Santa Fe Institute under the influence of Kenneth Arrow and the leadership of Brian Arthur. It advanced the view that the economy is a complex adaptive system, governed by the same dynamic principles that characterize other complex systems. In particular, Arthur’s work presents complexity economics as a general theoretical framework alternative to neoclassical economics. Then, the chapter analyzes the contributions of Robert Axelrod and other scientists at the University of Michigan, focusing on their studies on the complexity of cooperation, exploring how cooperation can emerge in a world of self-seeking egoists.