Dewatering may be defined as the displacement of fluid from a saturated porous medium. The mechanism of the process is not fully understood, but is known to be strongly dependent on the pore structure of the porous medium because of the action of capillary forces in the pores. This investigation attempts to describe the microscopic effects of moisture in porous media and to relate these to the macroscopic process of dewatering. This is divided into three parts: the first is concerned with the static effects of moisture retained in porous media, for which a model of pore space is developed; the second deals with the now of fluids in partially saturated porous media and an attempt is made to relate the model of pore space to this; finally a simple theory of dewatering is developed to demonstrate the effects of the various variables.