Since their appearance, from the first exhibition in London in 1851 to the future in Milan in 2015, we still wonder about the aim of these events. Their role changed, some skills lose their value, and others appeared. Few works focuses on this subject, of which we get sometime only the impetus for the host country to recreate her international image, when, on the contrary, those events become leading urban tools from one continent to another. Which are the principles which support them today? Who decides on their organization? Which regulations govern them? Which are the repercussions on the host town? Curiously, the great contemporary exhibitions have been little studied under these aspects. If the extent of the projects brings a certain scepticism about the real impact of these planning strategy, the principle of the exhibition is still seen as performing, crosses the XX century and is expressed in several countries. In order to question those subjects, it needs to wonder about them in the case of exhibitions that could not take place. Being not just a history of great exhibitions, this thesis, which first analyzes the questions on a general level, and then approaches it by cases of study, would like to fill an important gap on the methods of transformation of the cities which host these great events.