Since 2000, the governing body of world's fairs, the Bureau International des Expositions (BIE), has regularized how it awards expositions.
In years ending in "0" and "5," they allow for a "sanctioned" expo. Expo 2000, Expo 2005, and Expo 2010 were of this type. The planned Expo 2015 in Milan, Italy will also be this kind of exposition. They are the larger of the two types of world's fairs, analogous to the Summer Olympics. There are no size limitations for sanctioned world's fairs and many nations build their own stand-alone pavilions. Themes for these pavilions are fairly broad and typically have several smaller sub-themes.
In between each of these sanctioned world's fairs (but not in an adjacent year) there is the more modestly sized world's fair of the two. It can be thought of as analogous to the Winter Olympics. It is considered "recognized" by the BIE, meaning that it has a limited site size and national pavilions are usually exhibited in a modular system provided by the expo authorities. Themes for recognized expositions are far more specific than those of sanctioned expos. Expo 2008, Expo 2012, and the upcoming Expo 2017 in Astana, Kazkahstan follow this smaller model. It is generally thought that recognized expositions are ideal for smaller cities or cities in developing nations.
There was no world's fair held between Expo 2000 and Expo 2005. Kuala Lumpur was originally awarded Expo 2002, but bowed out. In 1997, Australia's Gold Coast was awarded the slot, but later government pressure cancelled their efforts.
Given these issues, the Paris suburb of Seine-Saint-Denis was awarded the right to host Expo 2004 despite the fact that the year was adjacent to that of Expo 2005's. A government change in 2002 prompted its cancellation.
Kuala Lumpur, Malaysia
(proposed for 2002, initial winner)
Gold Coast, Queensland, Australia
(proposed for 2002, secondary winner)
(proposed for 2004, tertiary winner)