From the IMO home page:
The International Mathematics Olympiad (IMO) is an annual mathematics competition for highschool students. It is one of the International Science Olympiads. The first IMO was held in Romania in 1959. The usual size of an official delegation to an IMO is (a maximum of) six student competitors and (a maximum of) two leaders. There is no official ``team''. The student competitors write two papers, on consecutive days, each paper consisting of three questions. Each question is worth seven marks.
You can check results and other info at
http://www.win.tue.nl/win/ioi/imo/