The discovery of quantum mechanics is considered to be one of the greatest achievements of mankind in the field of science. This theory is propounded by some of the eminent scientists, namely, Planck, Einstein, Bohr, Compton, Heisenberg, Dirac and others. It is a branch of Physics. This book, in a simple but effective style, presents the different stages of development of quantum theory starting from Planck’s law to De Broglie hypothesis. The book is intended for undergraduate and postgraduate students of Physics and Chemistry.