The process of sintering by volume diffusion has been subjected to a critical review. Various stages in the sintering process have been identified. It has been shown that sintering is most effective when the porosity takes the form of continuous pore channels along three—grain edges. The conditions determining when this stage finishes have been derived, and the effect of the subsequent stage on the sintering kinetics has been predicted. The available kinetic data for the sintering of BeO have been reviewed.