Because of recent experiments (H. Lakrout et al., J Adhes 1999, 69, 307) conducted with an instrumented probe-tack device, a better description of the debonding mechanisms of soft viscoelastic adhesives from a hard interface can be proposed. Because the deformed volume is about the magnitude of the sample size and the deformation is highly inhomogeneous, adhesion cannot be studied independently of the geometry of the test. One of the simplest geometries is that of the debonding of a cylindrical flat-ended probe from a thin film. In this case, a uniform displacement field is applied over the entire top surface of the film, and the stress field at the probe-film interface is strongly dependent on the degree of confinement of the film characterized by the ratio between the probe radius a and the film thickness h. For a very confined film, the nucleation of interfacial cavities is predicted to occur over most of the surface of the probe. These cavities will grow in the plane of the film and can be assimilated to multiple penny-shaped interfacial cracks. We present a semiquantitative model of how the lateral growth of these cavities is controlled by the competition between a critical energy-release rate at the interface G(c) and the elastic modulus of the film E. Finally, either these cavities coalesce and the film is debonded from the surface or the walls between these cavities become elongated in the tensile direction and form a fibrillar structure. The conditions for the formation of this fibrillar structure are qualitatively discussed.