Understanding clathrate hydrate formation and stability is an urgent issue due to their environmental importance. However, a fundamental understanding of the molecular processes governing their formation has remained elusive. We have now developed a novel method of preparing amorphous solutions, which crystallize slowly to cubic structure II clathrate hydrates upon heating in the temperature range 160 - 200 K. In this temperature range we were able to follow by powder X-ray diffraction how one or two unit cells form from the amorphous solution and grow to a crystal of the size of many unit cells. This finding now provides the unique opportunity of studying how the process of clathrate nucleation and growth takes place on the molecular level. This requires study of a set of isotopically substituted amorphous solutions and extraction of partials from neutron data.