A new, simple theoretical model to characterise the average continuum behaviour of the spall process is presented. Unlike all previous models, it is based on a non-local view of spallation and is applicable to both ductile and brittle materials. The model was incorporated into one and two dimensional hydrodynamic code to predict the free surface velocity of ductile and brittle targets in planar plate impact experiments, and can be extended to three dimensional problems. The results compared well with experimental data. The model can be used with common equations of state and constitutive equations.