We end the part of the book devoted to unit operations with this chapter, which discusses humidification and dehumidification processes, focusing on the mass and energy transfer between a gas and a pure liquid in which it is practically insoluble. The air/water system is examined in detail, with definition of key hygrometric quantities such as absolute, saturation, percentage, and relative humidity. Psychrometric charts and analytical relationships for enthalpy are introduced as essential tools for system analysis. The design and operation of cooling towers are analysed, including the determination of minimum gas flux, packing height, section and interfacial conditions using the two-film theory.