A world's fair, world fair, universal exposition, or world expo (expo short for exposition) is a large public exhibition. These exhibitions vary in character and are held in varying parts of the world.

The main attractions at world's fairs are the national pavilions, created by participating countries. At Expo 2000 Hanover, where countries created their own architecture, the average pavilion investment was about €13 million.^{[citation needed]} Given these costs, governments are sometimes hesitant to participate, because benefits are often assumed not to outweigh the costs. Tangible effects are difficult to measure, but an independent study for the Dutch pavilion at Expo 2000 estimated that the pavilion (which cost around €35 million) generated around €350 million of potential revenues for the Dutch economy. It also identified several key success factors for world-exposition pavilions in general.

Since the entering into force of the 1928 Convention Relating to International Exhibitions, the Bureau International des Expositions (BIE; English: International Exhibitions Bureau) has served as an international sanctioning body for world's fairs. BIE-approved fairs are of three types: universal, international, and specialized. They usually last from three weeks to six months.