Spacetime Foam and the Cosmological Constant

Sixty-five years ago, John Wheeler suggested that Planck-scale quantum fluctuations of geometry and topology---``spacetime foam''---might be important for understanding the cosmological constant. Twenty years later, Stephen Hawking initiated an exploration of this proposal through Euclidean path integral techniques. Here, I report on further progress, based on a canonical approach to quantum gravity and recent advances in the initial value formalism, that suggests that spacetime foam may indeed be capable of ``hiding'' a large cosmological constant.