Mathematics Problems
The International Mathematical Olympiad (IMO) is the World Championship Mathematics Competition for High School students and is held annually in a different country. The first IMO was held in 1959 in Romania, with 7 countries participating. It has gradually expanded to over 80 countries from 5 continents. You can find lots of mathematical problems here, most of them are not easy to handle. You can also link to IMO sites of different countries, including Hong Kong.

Try your hand at these Mathematical Talent Search problems and mail in your answers! You don't have to have solved all the problems. There are 5 Rounds each with 5 questions. Questions are very challenging.
This is a web-base of interesting mathematics problems that may be used in classrooms or just for fun. There are more than 100 questions with solutions.
Database of 20000 math problems from numerous journals and contests. Founded by MathPro Press and the University of Missouri - Rolla.
Rice Math Tournament
The Rice University Mathematics Tournament is an annual event for high school students and teams from schools in the Texas/Louisiana area. Sample problems from previous tournaments are given.


Examination papers from different Universities, mostly in Chinese.

Lots of problems with answers and solutions.
The Null Set
Mathematics Club of Princeton University. There are five sets of Puzzles ranging from Simple Puzzles to Impossible Puzzles.

Discussion of different types of problems