Cooling towers, evaporative fluid coolers and evaporative condensers are all members of a basic heat exchanger family. Heat is rejected by evaporation, from a gravity drained water film, into air flowing through a cooling tower "packing", or a tube bundle for fluid coolers and condensers. Hence, the air side heat and mass transfer process is governed by the same basic process. The key difference in the theory for each type relates to the thermal resistance of the process fluid. This resistance is quite small for cooling towers, but must be accounted for in the fluid cooler and condenser. This paper shows that a "unified" theoretical treatment may be applied to all three evaporative exchanger types. Specific equations or correlations for calculation of the heat and mass transfer resistance are discussed. In interest of preserving simplicity, the approximate Merkel equation is employed for the air side heat and mass transfer. The specific approximations associated with the Merkel equation are also noted. The theory presented in this paper provides the basis for computer simulation algorithms for cooling towers, fluid coolers and evaporative condensers.