There were a few experimental observations, towards the end of nineteenth century that could not be explained on the basis of the then well-established classical concept/mechanics. One was the discrete energy spectrum of hydrogen atom and the other was the intensity distribution of black body radiations. In 1900, Planck came forward with an innovative idea of discrete energy levels of harmonic oscillator, and was able to explain black body energy distribution. In 1913, Bohr put forward his atom model of discrete values of angular momentum of electrons, and was able to explain many features of discrete energy spectrum of hydrogen atom. Here we review, in brief, these early developments.