The Advanced Study Institute on 'Elementary Excitations in Solids, Molecules, and Atoms' was held at the University of Antwerp (U.I.A.) from June 18th till June 30th 1973. The In stitute was sponsored by NATO. Co-sponsors were: Agfa-Gevaert N.V. (Mortsel - Belgium), Bell Telephone Mfg. Co. (Antwerp Belgium), the National Science Foundation (Washington D.C. - U.S.A.) and the University of Antwerp (U.I.A.). A total of 120 lecturers and participants attended the Institute. Over the last few years, substantial progress has been made in the description of the elementary excitations of the elec tronic and vibrational systems and their interactions. Parallel with this, the experimentalists have obtained outstanding re sults, partly as a result of availability of coherent light sour ces from the far infrared through the visible region, and partly because of the availability of synchrotron radiation sources in the soft X-ray region. The results of today will lead to fur ther progress over the next years. It was the purpose of this NATO Advanced Study Institute to present astate of the art, namely a survey of experiment and theory.

Inhalt
of Part A.- Plasmons.- Many-Body Effects at Surfaces.- Inelastic X-Ray Scatterings from Electrons in Matter.- to Collective Excitations in Solids.- Plasmons in Inhomogeneous Electron Gases.- Electronic Structure in Crystals.- Hartree-Fock Model Calculation for Atoms and Crystals plus Corrections to the Hartree-Fock Excitation Energies.- Ab Initio Energy Band Methods.- Effects of Exchange and Correlation in the Electron Bandstructure Problem.- Many-Body Perturbation Theory for Non-Uniform Electron Systems.- Atoms Molecules and Liquids.- Many-Electron Effects in Spectra of Electrons Bound in Atoms and Solids.- Soft X-Ray Spectra of Molecules.- Collective Excitations of Dipolar Liquids.- Seminars.- Dynamical Image Charge Interactions.- Fine Structure Correction of the Edge Singularity in the X-Ray Spectra of Metals.- Author Index to Part A.- Subject Index to Part A.