Molecular simulations of yield in models of glassy polymers and amorphous metals are used to test classic yield criteria. Bulk systems follow the pressure-modified von Mises criterion, which is based on an isotropic, energy-driven model of yield. Thin films follow the {Mohr–Coulomb} criterion, which is based on an anisotropic friction model. The difference in behavior is correlated with a change in shear mechanism, and the connection between thin film results and macroscopic friction laws is discussed.