Are there problems tougher than Poincare conjecture?

I read this news article [ http://www.merinews.com/article/grigory-perelman-russian-mathematician-refused-1-million-prize-for-solving-century-old-puzzle/15825038.shtml ] about Grigori Perelman who solved "one of the most important open questions in topology". The article also says that " The... show more I read this news article [ http://www.merinews.com/article/grigory-perelman-russian-mathematician-refused-1-million-prize-for-solving-century-old-puzzle/15825038.shtml ] about Grigori Perelman who solved "one of the most important open questions in topology". The article also says that " The Poincare conjecture is the first and only solved Millennium problem till date."
I have some keen interest in mathematics and I would really like to know which are those other unsolved "Millennium" problems.
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