Spacetime foam

Spacetime foam is analyzed within the simplistic model of a set of scalar fields on a flat background. We suggest the formula for the path integral which allows to account for the all possible topologies of spacetime. We show that the proper path integral defines a cutoff for the field theory. The form of the cutoff is fixed by the field theory itself and has no free additional parameters. New features of the Feynman diagram technic are outlined and possible applications in quantum gravity are discussed.