Four compressibilities are defined for porous rock, relating changes in the bulk and pore volumes to changes in the pore and confining pressures. Using a micromechanical theory based on classical linear elasticity, three relations are found between these compressibilities. Two of these relations are verified experimentally for Berea and Bandera sandstone. Bounds are derived for these compressibilities, involving only the porosity and the elastic moduli of the rock-forming minerals. For the strains to be unique functions of the stresses the compressibilities must be functions only of the difference between the confining and pore pressures. This dependence is verified for Berea, Bandera, and Boise sandstone. While the strains cannot be expressed as functions of an ''effective stress,'' regardless of how it is defined, the (elastic) volumetric behavior of porous rocks can be studied through tests conducted at zero pore pressure.