We discuss the question of frame equivalence of scalar-tensor theories in the path integral approach. We find that due to diffeomorphism invariance of the background geometry, the path integral measure must depend non-trivially on the metric. Under a change of frames, this induces new finite pieces in the Quantum Effective Action that must be taken into account to preserve frame equivalence at the quantum level. These pieces might be of relevance to asses the dynamics of the Early Universe within certain inflationary models.