The elementary building bricks of matter have long been known: the elec tron (1897), the proton (1911) and the neutron (1932); in addition, there was the quantum of radiation, the photon (1905), known to have a double nature with both wave and particle features. The bold idea (L. de Broglie, 1924) that streams of ordinary particles such as electrons should possess a similar double nature led to the development of wave mechanics; moreover, diffraction phenomena were found with beams of electrons, atoms and even molecules. Pauli's Exclusion Principle causes us to distinguish between fermions...