Filters are used in geotechnical engineering to control seepage and to prevent erosion of soil due to the drag forces of seeping water. Filters act as barriers to retain the base soil while allowing seepage flows to exit without causing high hydraulic gradients or pore pressures which may damage the structure. This paper describes a new anyalytical model of filtration. The model is based on a three dimensional network model of filter pores, and the equations of conservatioln of mass and momentum which govern the rate of particle transport. The model has application in the design of granular filters for protecting non-cohesive base soils in embankment dams, retaining walls, drainage wells or road pavements.