In what follows, I examine three main points which may help us to understand the deep nature of Einstein's objections to quantum mechanics. After having played a fundamental pioneer role in the birth of quantum physics, Einstein was, as is well known, far less enthusiastic about its constitution as a quantum mechanics and, since 1927, he constantly argued against the pretention of its founders and proponents to have settled a definitive and complete theory. I emphasize first the importance of the (...) philosophical climate, which was dominated by the Copenhagen orthodoxy and Bohr's idea of complementarity: What Einstein was primarily reluctant to was to accept the fundamental character of quantum mechanics as such, and to modify for it the basic principles of knowledge. I thus stress the main lines of Einstein's own programme in respect to quantum physics, which is to be considered in relation to his other contemporary attempts and achievements. Finally, I show how Einstein's arguments, when dealing with his objections, have been fruitful and some of them still worthy, with regard to recent developments concerning local nonseparability as well as concerning the problems of completeness and accomplishment of quantum theory. (shrink)