The Holographic principle, in the form of the AdS/CFT correspondence, suggests a relation between processes in quantum gravity and phenomena in ordinary quantum field theory. The old `sum over histories' semi-classical approach to quantum gravity can be revisited in this light, by studying various gravitational instantons in AdS. We show that the AdS/CFT correspondence may be extended to spaces which are only locally asymptotically AdS, by examining the properties of the AdS-Taub-NUT and AdS-Taub-Bolt spacetimes. We also use holography to show that spacetime topology change in quantum gravity is a unitary process, in contrast to suggestions in the old `sum over histories' literature.