The contribution of the image potential to the transmission through a potential barrier is examined. Results of measurements of semiconductor tunnel barriers are presented which provide evidence for a transition to a dynamic regime (and a reduced contribution of the image) as the tunneling time through the barrier becomes shorter. These results are related to the work of Büttiker and Landauer on the tunneling time, and to the Persson-Baratoff theory of the dynamic image potential. Possible implications of these concepts for the dynamics of the Coulomb blockade are briefly mentioned. © 1992.