This compilation opens with an exploration of the vadose, or unsaturated zone, which is of utmost importance as the nexus between surface water and groundwater. It is the link between what happens on the surface and what happens below, inside the aquifers. As such, understanding this underground natural environment is essential for the sustainable development of society. Due to the complexities involved in considering the hysteretic response of the Soil Water Characteristic Curve and its dependency on volume changes, these two features are often ignored in numerical studies of unsaturated soils. To facilitate their use in numerical modeling, a model for the Soil Water Characteristic Curve equation based on the bounding surface concept is proposed. The authors go on to focus on the temperature distribution and water migration in unsaturated soil, which is of significant importance because these factors are related to the thermal-physical properties of soil. The changes in these properties may cause disastrous engineering problems such as the cracking of pavement, damage of structure foundation and fracture of pipelines. Lastly, the book addresses the measurement and expression of the mass chemical potential in the presence of superficial layers induced by a complex structure. By definition, the mass chemical potential of a constituent in a soil represents the variation of the internal energy of the medium when a unit mass of the constituent is transferred to a reference state with the entropy, volume and mass of the other constituents remaining constant.