Principles of Modern Physics covers important developments in physics during the twentieth century. Beginning with the development of the quantum concept and radiation laws, followed by Einstein's special relativity, it covers atomic structure, basics of spectra, basic (non relativistic) quantum mechanics with an introduction to Dirac's relativistic wave equation and the problem of hydrogen atom. This follows the statistical distribution laws, X-rays and physics of solids, their imperfections, magnetic properties and superconductivity (including newly discovered high Tc superconductors), Zeeman and Stark effects, Lasers, nuclear physics, radio-activity, nuclear fission and fusion, particle accelerators and detectors. It features a discussion on Universe (including stellar evolution Chandrasekhar limit, black holes and big-bang theory), elementary particles (including tau-theta puzzle, SU(2) and SU(3) symmetry, the Eightfold- way, ...