  Various experiments evidence spatial heterogeneities in sheared yield stress
fluids. To account for heterogeneities in the velocity gradient direction, we
use a simple model corresponding to a non-monotonous local constitutive curve
and study a simple shear geometry. Different types of boundary conditions are
considered. Under controlled macroscopic shear stress $\Sigma$, we find
homogeneous flow in the bulk and a hysteretic macroscopic stress - shear rate
curve. Under controlled macroscopic shear rate $\dot{\Gamma}$, shear banding is
predicted within a range of values of $\dot{\Gamma}$. For small shear rates,
stick slip can also be observed. These qualitative behaviours are robust when
changing the boundary conditions.
