This is an 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 volume concentrates on the topic of allocation information and markets. The foundations of most of neoclassical economics can be seen to be the concept of competitive equilibrium, which stems from the interaction of an economy-wide system of markets. In the 1950s, the Arrow-Debreu model of general equilibrium provided a new point of departure for such analyses and led naturally to an examination of the assumptions lying behind them, in particular to questions of how information and allocation interact. These essays aim to demonstrate how economists currently formulate and seek to answer these questions.