We study the undulatory instability of a straight crack front generated by peeling a
flexible elastic plate from a thin elastomeric adhesive film. We show that there is a
threshold for the onset of the instability that is dependent on the ratio of two lengthscales that arise naturally in the problem: the thickness of the film and an elastic length
defined by the stiffness of the plate and that of the film. A linear stability analysis
predicts that the wavelength of the instability scales linearly with the film thickness. Our
results are qualitatively and quantitatively consistent with recent experiments, and show
how crack fronts may lose stability due to a competition between bulk and surface effects
in the presence of multiple length scales.