Unsaturated geomaterials are those geomaterials where the void spaces are partially filled with liquid and partially filled with gas. The liquid (wetting) phase is an aqueous solution generically referred to as water whereas the gaseous (nonwetting) phase is a mixture of air and water vapour generically referred to as air. The mutual interaction between these two fluid phases and their interaction with solid phase play a key role in the mechanical and hydraulic response of unsaturated geomaterials. The basic mechanisms and thermodynamics of the interaction between the liquid, gaseous, and solid phases are not commonly covered by undergraduate and graduate courses. As a result, students and engineers with geotechnical background may find it difficult to approach the mechanics and hydraulics of unsaturated soils. The purpose of this chapter is to fill this gap and to illustrate the basic elementary mechanisms behind water retention, water flow, and mechanical behaviour of unsaturated geomaterials. Special emphasis is given to capillary mechanisms arising from surface tension at the air-water interface and from the angle formed by the air-water interface at the solid-liquid-gas junction (contact angle). Capillary actions play a major role in the response of unsaturated geomaterials and can conveniently serve as a basis to introduce the most distinctive features of the hydraulic and mechanical response of unsaturated geomaterials.