Ever since its birth following the introduction of Planck's constant h in 1900, quantum theory has been legitimated only by the amazing capacity it has exhibited in accounting quantitatively for a huge variety of physical phenomena. The so-called Copenhagen interpretation has allowed to accomodate in a coherent frame the tenets of quantum mechanics, but at the cost of bringing in the demand of abandoning, at the microscopic level, causality, determinism, and physical reality. The results of Aspect and coworkers' ingenious experiments, however, can give further support to the belief in the unlimited validity of the quantum theory. But, for all its successes, the theory remains basically incomprehensible.