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.

De Broglie's initial conception of De Broglie waves.- Wave-particle duality of light: a current perspective.- Wave-particle dualism in matter wave interferometry.- Neutron wave optics studied with ultracold neutrons.- "Gespensterfelder".- On the possibility of realising a low intensity interference experiment with a determination of the particle trajectory.- From ghost to real waves: a proposed solution to the wave-particle dilemma.- Permanence of the corpuscular appearance and non linearity of the wave equation.- Could solitons be adiabatic invariants attached to certain non linear equations?.- of the chronon in the theory of electron and the wave-particle duality.- The wave-particle duality as an interplay between order and chaos.- Structures in semiclassical spectra: a question of scale.- Ray optics for diffraction: a useful paradox in a path integral context.- Atomic collision experiments at the border line between classical and quantum mechanics.- On the superposition principle and its physics content.- The probabilistic roots of the quantum mechanical paradoxes.- Causality and symmetry.- A critical analysis of the quantum theory of measurement.- Quantum interference of probabilities and hidden variable theories.- Experimental tests of Bell's inequalities.- Tests of the non separability of the $$
{{\rm{K}}^{\rm{0}}}{{\rm{\bar K}}^{\rm{0}}}
$$ system.- A new approach to testing the separability in microphysics: Rapisarda's experiment.- On the possibility of extending the tests of quantum mechanical correlations.- How do we have to change quantum mechanics in order to describe separated systems?.- On the four-dimensional character of micro-physical phenomena.- Dynamics of the reduction of the statevector.- CPT revisted: a manifestly covariant presentation.-Wave-particle duality in a quark model.- Destruction of coherence in nondemolition monitoring: quantum "watchdog effect" in gravity wave detectors.- Nonlocal hidden variables and nonlocal gauge theories.- A finite particle number approach to physics.