Polymer/polymer interfaces reinforced with block copolymers or grafted chains can fail by the pull-out of these chains. A number of constitutive models have been proposed in the literature to describe the pull-out of a single chain. In this paper, we use these models to construct a continuum theory of a weak interface, and we study the stability of such an interface subjected to a homogeneous deformation. We show tht if the perturbation contains a Fourier component with wavelength exceeding wa critical value Lambda(c), then the homogeneous deformation is unstable. The dependence of Lambda(c), on the geometry, the stiffness of the material outside the interface, and the pull-out models is derived. We also study the stress field near the tip of a preexisting interface crack using the different pull-out models.