The suggestion that quantum-mechanical tunnelling might be a significant factor in some chemical reactions was first made fifty years ago by Hund, very soon after the principles of wave mechanics had been established by de Broglie, Schrodinger and Heisenberg, and similar ideas were put forward during the following thirty years by a number of authors. It was realised from the beginning that such effects would be particularly prominent in reactions involving the movement of protons or hydrogen atoms, and both theoretical and experimental work received a powerful stimulus in the discovery of...