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.

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.