Abstract: Based on Hertz′s diary, letters and collected works we analyse his major works on the theory of the classical electromagnetic field. We attempt to clarify some misunderstandings about his achievements in this field, giving his role in the development of classical electromagnetism a more objective evaluation. His theoretical research from 1884 to the early 1890′s has important significance in the history of physics. Not only did Hertz promote Maxwell′s theory throughout Germany, but also reconstructed and expanded the theoretical foundation of Maxwell′s equations. Furthermore, from analysis of Lorentz′s works on electromagnetism and Einstein′s work in the earlier days of his life, we show that Hertz′s achievements also had a significant influence on Lorentz′s electronics and Einstein′s early physics studies.