This is an account of Einstein's work and thoughts on the quantum theory. The following topics will be discussed: The light-quantum hypothesis and its gradual evolution into the photon concept. Barly history
o r t h e p h o t o e l e c t r i c e l e c t m e c h e o r ec a a n d e x p e r i m e n t a r e a s o n s
stronger and more protracted than for any other particle proposed to date.