This chapter deals with the properties of soil water which should be considered in any comprehensive theory of its behavior. The water present in the soil in its various stages of wetting has been described functionally by the terms hygroscopic, capillary, and gravitational water, and geometrically by the terms pendular and funicular water. It has been found that there are functional relationships between the water content and physical variables such as vapor pressure, matric suction and capillary or hydraulic conductivity. The forces acting on the soil water can be classified, for convenience, into matric forces (those which result from the presence of the solid phase), osmotic forces (those caused by dissolved solutes), and body forces (inertial forces and gravitational force). Calculations of energy relationships and potential distributions in the soil furnish the best means yet devised for explaining and predicting by a single theory most of the phenomena relating to soil water.