Millennium Prize Problems in Economics?

In my previous post, I mentioned the book by Marcus Du Sautoy on the history of prime numbers. Besides the main topic, this book also evokes my thinking on another thread. 

You guys may know the famous 1899 Hilbert's 23 unsolved problems. It is then followed by Clay Institute's Millennium Prize Problems. So if there is a counterpart for economists, what would it like to be? 

It may be harder to figure out compared with its mathematical cousin since economic problems are not so well defined given their close connections to real world problems. Assumption, modeling and solution are equivalently important.