In 1905, Einstein published three of the greatest papers in physics on the subjects of Brownian motion, special relativity and light quanta. The third of the series is the most revolutionary of the three and asserts that, under certain circumstances, light can be considered to consist of particles rather than waves. Using his deep understanding of entropy and statistics, he showed that, in the Wien region of the spectrum, light behaves as if it consisted of light quanta with energies hn. He then showed that, if Planck had followed the proper Boltzmann procedure, he would have obtained the Planck formula for black-body radiation. He proposed three test of his hypothesis, the most important being the energy dependence of the photoelectric effect upon the frequency of the incident radiation. He could also account for discrepancies between the classical and quantum expressions for the specific heat capacities of gases.