Interfaces are a key element in ceramic matrix composites, but also in several material assemblies, such as, for instance, multilayers. This is not only because they can cause crack deflection and make materials damage tolerant, but also because they can be tailored with regard to material performances. An approach to crack deflection at interfaces or within interphases is proposed on the basis of the following Cook and Gordon’s mechanism: a crack is nucleated along an interface, ahead of a propagating crack; deflection of this crack then results from coalescence with the interface crack. The stress state induced by a crack was computed in a cell of bimaterial using the finite element method. The cell represents a matrix and a fiber, or an interphase and a fiber or two layers in a multilayer. A master curve was established. It represents the debonding condition based on strengths and elastic moduli of constituents. Then a deviation potential was defined. Deviation potentials were calculated for various fibre/matrix or layer combinations.