The amount of water existing in the soil mass significantly influences the engineering behavior of soil because the water content will affect the state of stress of the soil under a load. Other behavior related to water includes densification (compaction) properties, shrinkage, swelling, settlement, and progressive failure. In this chapter, discussions focus on capillary, hydraulic conductivity, and mass transport phenomena. Applications emphasize the areas of ground improvement engineering and the soil decontamination process. Other topics such as shrinkage, swelling, densification, settlement, and strength are discussed in Chapters 6 and 11.