The Bureau International des Expositions (BIE) was founded in 1928 and regulates international expositions as well as other exhibits on behalf of more than 100 member nations.
Not all events that most people would categorize as a world's fair have been sanctioned by the BIE, notably the 1964-1965 New York World's Fair.
Currently, the BIE "sanctions" large world expositions that are held in years ending in "0" and "5." The organization "recognizes" smaller international expositions that are allowed to happen once in between each larger expo (but not in an adjacent year).
In the coming years, the BIE will decide who will host Expo 2020 as well as the smaller Expo 2017.