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The history of electron emission is reviewed from a standpoint of the work function and the applications. For years, in the field of thermionic emission, a great deal of efforts have been devoted to search for low work function materials with high melting temperature, while the reduction of the local change in time of the work function rather than the work function itself has been the main issue of field emission researches. High brightness and long life are the central targets of the emission material researches for scientific instrument application, while high current density and low power consumption are the guiding principles for display application. In both cases, field emission has been dominating the applications recently. In all above cases the main issue in the future researches will be to analyze the work function in atomic level and thereby to understand the mechanism of the work function reduction by atom adsorption, the change in time of the local work function leading to the current fluctuation, and the relationship between microscopic and macroscopic work functions. Our attempt is discussed, where the work function in atomic level is measured by utilizing the STM-technique and it is made clear how far the work function in atomic level extends its influence over the neighboring sites. As a result, a simple relationship is established between microscopic and macroscopic work functions.