Over the past several decades, due to the rapid development of science and technology and the ever-increasing groundwater extraction and contamination, many new theories, methodologies and applications regarding hydraulic conductivity have emerged. Water can move through soil as saturated flow, unsaturated flow, or vapor flow. Saturated flow takes place when the soil pores are completely filled (or saturated) with water. It depends on the intrinsic permeability of the material, the degree of saturation, and on the density and viscosity of the fluid. Unsaturated flow occurs when the larger pores in the soil are filled with air, leaving only the smaller pores to hold and transmit water. Vapor flow occurs as vapor pressure differences develop in relatively dry soils. Vapor migrates from an area of high vapor pressure to an area of low vapor pressure. Hydraulic conductivity is a soil property that describes the ease with which the soil pores permit water (not vapor) movement. It depends on the type of soil, porosity, and the configuration of the soil pores. Hydraulic conductivity is probably the most important hydrogeological parameter, as it governs, along with other parameters, the flow of fluids and migration of contaminants beneath the ground surface, especially in soils and aquifers. The book Hydraulic Conductivity - Issues, Determination and Applications presents comprehensive reviews of new measurements and numerical techniques for estimating hydraulic conductivity.