Photoelectron spectroscopy has matured considerably during the last decade. The experimental techniques were improved impressively, and a deeper theoretical insight into the underlying mechanisms of photoemission could be achieved. It seemed that it was time to take stock of what has been accomplished so far. The collection of tables and diagrams given in this volume is a contribution to a larger programme aiming at a critical, and as comprehensive as possible, tabulation of electronic structure information obtained by electron and photon spectroscopy. No such collection of photoelectron spectra and related data exists at present, and the editors hope to fill a need of both experimentalists and theoreticians in various fields of solid state research. The policy for this volume was to offer, besides the photoelectron results, also a limited set of other data (like lattice constants and work functions) useful in the context of band structure information. With regard to the amount of available data and in order to speed up publication, it seemed necessary to divide volume III/23 into two subvolumes a and b. The chapters of the present volume III/23a have been printed as each author completed his contribution. The order of the chapters is thus chronological rather than thematic. Subvolume b is in preparation and will appear in due course.