The purpose of the study is to characterize the main features of the anisotropic mechanical behaviour of a shale (Opalinus Clay), and to identify an anisotropic constitutive framework adapted to the response of the material under mechanical perturbations. Such a constitutive model is required for the simulation of triaxial tests which were done on OC samples. Firstly, the large quantity of available laboratory tests which characterize the mechanical response of OC has been analyzed. The test series provide clear evidence for anisotropic mechanical behaviour of Opalinus Clay. It appears that the stress history and the micro-structure of the material induce a typical response of the material which is more overconsolidated (higher rigidity, less ductile behaviour…) for loading direction parallel to the bedding plane than perpendicular to it. Concerning numerical investigations, the capabilities of the model of Hujeux have been assessed in reproducing the anisotropic features of behaviour of OC. The constitutive model uses the theory of multi-mechanisms plasticity. Two fundamental observations account for the adequacy of the chosen constitutive framework: the elastic moduli depend on the direction of shearing, and induced anisotropy will affect the way the material hardens or softens depending on the angle of shearing.