Many different expressions have been proposed for the effective stress principle for porous media, but none has clearly been acknowledged as being the correct one. It should be possible to determine the correct expression, because this is fundamental principle of mechanics. Following review of the candidate expressions proposed for soil, concrete and rock, a comprehensive expression is derived involving the factors observed in experiments on 'artificial rock'. In this new expression a distinction is made between the compressibilities of grains and skeleton due to total stresses and pore pressures. This general and more comprehensive expression is then tested against the experimental evidence. The expression may be specialized for (a) granular materials with separate grains with contact points and (b) solid rock with interconnected pores. The new expression for the effective stress contains most of the previously proposed expressions as special cases. Based on results of compression tests on quartz (with h...