  Among seven problems, proposed for XXI century by Clay Mathematical
Institute, there are two stemming from physics. One of them is called
"Yang-Mills Existence and Mass Gap". The detailed statement of the problem,
written by A. Jaffe and E. Witten, gives both motivation and exposition of
related mathematical results, known until now. Having some experience in the
matter, I decided to completement their text by my own personal comments aimed
mostly to mathematical audience.
