A paradox in understanding protein folding that was pointed out by Cyrus Levinthal (1922–90) in 1969. Levinthal observed that, since proteins have large numbers of atoms, the numbers of conformations they might have are very large indeed. From this it might appear that the time required by an unfolded protein to find the correct conformation would be much longer than the age of the universe; yet the time actually taken is a fraction of a second. Several resolutions of this paradox have been proposed but it appears that the use of the energy landscape concept in protein folding gives the most satisfactory resolution.