This chapter deals with the steady movement of water in soils. It discusses water flow in soils and its importance to engineering structures. In earth dams, for instance, the geotechnical engineer wishes to know the amount of water that will be lost due to seepage through the embankment dam and its foundation; in an industrial waste disposal pond, one wishes to create a nearly impervious soil barrier to avoid ground water contamination. Therefore, the engineer must select the appropriate soil type that has the required permeability. The permeability of soils can be measured through laboratory and in situ tests. In slope stability analysis, using total unit weights for soils and boundary forces is generally recommended. The chapter covers the most common types: the laboratory tests with constant or variable head permeameters. It also discusses liquefaction and hydraulic gradient determination.