1. S.W. Hawking, Space time foam, DAMTP preprint.
2. Gravitational instantons
3. G.W. Gibbons and S.W. Hawking, Classification of gravitational instanton symmetries, DAMTP preprint.
4. G.W. Gibbons, S.W. Hawking and M.J. Perry, Path integrals and the indefiniteness of the gravitational action, DAMTP preprint, Nucl. Phys. B, to be published.