  We review here the main contributions of Einstein to the quantum theory. To
put them in perspective we first give an account of Physics as it was before
him. It is followed by a brief account of the problem of black body radiation
which provided the context for Planck to introduce the idea of quantum.
Einstein's revolutionary paper of 1905 on light-quantum hypothesis is then
described as well as an application of this idea to the photoelectric effect.
We next take up a discussion of Einstein's other contributions to old quantum
theory. These include (i) his theory of specific heat of solids, which was the
first application of quantum theory to matter, (ii) his discovery of
wave-particle duality for light and (iii) Einstein's A and B coefficients
relating to the probabilities of emission and absorption of light by atomic
systems and his discovery of radiation stimulated emission of light which
provides the basis for laser action. We then describe Einstein's contribution
to quantum statistics viz Bose-Einstein Statistics and his prediction of
Bose-Einstein condensation of a boson gas. Einstein played a pivotal role in
the discovery of Quantum mechanics and this is briefly mentioned. After 1925
Einstein's contributed mainly to the foundations of Quantum Mechanics. We
choose to discuss here (i) his Ensemble (or Statistical) Interpretation of
Quantum Mechanics and (ii) the discovery of Einstein-Podolsky-Rosen (EPR)
correlations and the EPR theorem on the conflict between Einstein-Locality and
the completeness of the formalism of Quantum Mechanics. We end with some
comments on later developments.
