This is a extract from the 4-volume dictionary of economics, a reference book which aims to define the subject of economics today. 1300 subject entries in the complete work cover the broad themes of economic theory. This concentrates on the topic of game theory, aiming to offer a systematic explanation of strategic behaviour, providing economists with a set of powerful analytic tools that can be applied to many aspects of decision-making. The essays in this volume show how, and why, this is one of the most rapidly expanding areas of economic theory today as well as one of the most exciting and intriguing.