Expo (short for "exposition", and also known as World Fair and World's Fair) is the name given to various large public exhibitions held since the mid-19th century. The official sanctioning body is the Bureau International des Expositions (BIE), translated in English as the International Exhibitions Bureau (though sometimes rendered as the Bureau of International Expositions). BIE-approved fairs are divided into a number of types: universal, and international or specialized. They usually last between 3 and 6 months. In addition, countries can hold their own 'fair', 'exposition', or 'exhibition', without BIE endorsement.

Today, world expositions are the third largest event in the world in terms of economic and cultural impact, after the FIFA World Cup and the Olympic Games. They have been organized for more than one and a half centuries — longer than both the (modern) Olympic Games and the World Cup. The first Expo was held in The Crystal Palace in Hyde Park, London, in 1851 under the title “Great Exhibition of the Works of Industry of All Nations”. The “Great Exhibition” as it is often called was an idea of Prince Albert, Queen Victoria’s husband, and was the first international exhibition of manufactured products. As such, it influenced the development of several aspects of society including art and design education, international trade and relations, and even tourism. Also, it was the precedent for the many international exhibitions, later called “World’s Fairs”, which were subsequently held to the present day.

The main attractions at World's Fairs are the national pavilions, created by participating countries. At Expo 2000 Hannover, where countries created their own architecture, the average pavilion investment was around € 13 million. Given these costs, EU governments are sometimes skeptical about participation as tangible benefits are often assumed not to outweigh the costs. Effects are often not measured, however. An exception was an independent study for the Dutch pavilion at Expo 2000. This research estimated 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 .