We model the statics and dynamics of wetting of liquids on thin rubber films. We show that at equilibrium the capillary forces deform soft substrates, leading to undulations of the rubber film surface originating at the triple Line. Such deformations propagate when a liquid spreads on the rubber film. This induces the dissipation that may control the spreading dynamics. We derive a general expression for this dissipation as a function of the viscoelastic properties of the substrate. We calculate the spreading velocities for various mechanical relaxation models. Using available mechanical relaxation data, we find that our model may well account for the experimental observations of Carre and Shanahan, who demonstrated the importance of viscoelastic braking effects.