Valuable compounds often occur in nature in materials that contain water, such as fresh plants. Extraction with hot water has been carried out for many centuries at ambient pressure. Liquid water at temperatures T > 100 °C and pressures P > 0.1 MPa extends these applications and makes available compounds that have been not accessible at low-pressure conditions. At near-critical and supercritical conditions, the effective extraction of contaminants from waste materials becomes feasible. The extraction process characterized by solubility, kinetics, and mass transfer is treated in this chapter.