This thesis will deal with the historical and the physical aspect of Einstein's quantum of light paper from the year 1905. Light had been the center of observation and description since antiquity, there had been a number of theories about the nature of light which will be mentioned here. In the 17th century The corpuscular theory of light had been suggested in the 17th century and it managed to describe certain physical phenomena, but the phenomena that were noticed afterwards could not be described with this kind of theory. The idea of a corpuscular nature of light, with the ability to explain some new observations, dominated in the 19th century. Near the end of the 19th century it seemed that most observations had been successfully explained, however, several individuals wanted to explore the remaining uncertainties. One of the most important ones was Max Planck, who changed the course of the evolution of physics with the law of black body radiation. Albert Einstein continued to work on a similar problem and in his miraculous year published his revolutionary paper On a Heuristic Point of View Concerning the Production and Transformation of Light where he introduced the hypothesis of quantum of light and set a number of doings and discoveries which resulted in our present knowledge about the nature of light. This thesis describes the problems of physics at the end of 19th century and the attempts to solve them with respect to the experimental data. Firstly, the black body problem is considered, which is unexplainable using classical physics. Wien's radiation law and its agreement with experiments is elaborated, and the same is done for the Rayleigh-Jeans law. The work of Max Planck from the year 1900 is shown, where he mathematically derived the energy quantization of a linear harmonic oscillator. Then, the status which Einstein finds in that situation is described, including how he managed to explain Planck's math with an entirely new physical interpretation. All this leads to the analysis and explanation of his »heuristic« paper. His assumptions and formality which resulted in a completely new idea are examined. Reactions of many physicist to this new idea are stated and considered, including their adequate arguments for or against. It is shown that the path to the nature of light which we today know was long and sometimes hard-laboring up to the discovery of Compton's effect which succeeded in opening the door to accepting the quantum of light. Near the end, it is mentioned what consequences for science and technology the introduction of quantum of light had. At the very end, the entire work of Albert Einstein after the year 1905 is considered, especially his struggle with the quantum theory which lasted until the end of his life.