The Louis de Broglie Foundation (which was created in 1973, for the fiftieth anniversary of the discovery of wave mechanics) and the University of Perugia, have offered an international symposium to Louis de Broglie on his 90th birthday. This publication re­ presents the Proceedings of this conference which was held in Perugia on April 22-30, 1982. It was an opportunity for the developing of physical conceptions of all origins, which may serve to throw light on the mysterious power of the quantum theory. Quantum Mechanics has reached matu­ rity in its formalism and although no experiment yet has come to challenge its predictions, one may question the limits of its va­ lidity. In fact the true meaning of this vision of the microphysi­ cal world remains the subject of endless debating, at the heart of which lies "the foundational myth" of wave-particle dualism. Albert Einstein and Louis de Broglie are the two discoverers of this fundamental duality, which they always considered as a deep physical reality rather than a phenomenological artifice. During the conference a survey has been given of the essential recent experimental results in corpuscular and quantum optics and the most up-to-date theoretical aspects of the specificity of mi­ crophysical phenomena : various interpretations of quantum mecha­ nics, "al ternati ve theories" and hidden parameters theories, pro·­ babilistic and axiomatic questions and tentative crucial experi­ ments. The conference took place in the magnificent atmosphere of the villa Colombella lent to us by the Universita per Stranieri di Perugia.