Past research on evapotranspiration has provided sound theoretical knowledge and practical applications that have been validated through field measurements. Many different approaches have been used; however, when primary concepts and standard definitions are accepted, it is possible to find reasonable agreement among methods. This paper reviews such approaches, from Penman to Penman-Monteith. The standard concepts of potential evaporation (EP) and equilibrium evaporation (E_e), and the introduction of the climatic resistance (re), provide a better understanding of the role of the climate together with surface and aerodynamic resistances (r_s and ra). Therefore, the concept of reference evapotranspiration (ET_o), particularly the new one adopted by the Food and Agricultural Organization of the United Nations, can be better understood, as well as its limitations. Crop evapotranspiration (ET_c) is related to both ET_o and Ee. Crop coefficients (K_c) can be shown to have two components, α_o and α_c, with Kc = α_oα_c. The α_o is a function of the climatic resistance and of the aerodynamic resistances of the crop and of the reference crop. The α_c is a function of both surface and aerodynamic resistances of the crop and of the reference crop. From this analysis some ideas on future developments result that are directed toward providing compatibility between the one- and two-step calculation of ET_c.