Surface Science is understood as a relatively young scientific discipline, concerned with the physical and chemical properties of and phenomena on clean and covered solid surfaces, studied under a variety of conditions. The adsorption of atoms and molecules on solid surfaces is, for example, such a condition, connected with more or less drastic changes of all surface properties. An adsorption event is frequently observed in nature and found to be of technical importance in many industrial processes. For this reason, Surface Science is interdisciplinary by its very nature, and as such an important intermediary between fundamental and applied research. The present volume 42 is devoted to Covered Solid Surfaces and, in particular, Subvolume A to Adsorbed Layers on Surfaces. It is as such a collection of data obtained for adsorbates on well-defined crystalline surfaces. "Well-defined" means surfaces of known crystallographic structure and chemical composition.