Even the casual reader cannot fail to notice the somewhat uneven presentation of the contributions contributians to this volume, in particular what concerns the st style. yle. A closer scrutiny will also reveal that whereas the English language is certainly the preferred vehicle for commu- nication in astronomy, it is not the mother tongue of all contributors. However, while editing this volume I have felt that it would be more important to assure a speedy publication than to attempt to achieve a high degree of uniformity, which would anyhow be extremely diffi- cult with more than 100 eontributing contributing authors. When published, this book should stiIl still be a tool for aetive active research, not a museum pieee. piece. I am grateful to the organizers and editors of the individual sections seetions for having produced produeed their parts with within in the allotted time, and with a high degree of professionalism. A special speeial word of thanks goes to my eollaborators collaborators at the European Southern Observatory, Mrs. E. Volk, Volk, Mr. Nr. C. Madsen, and Mr. J. _Leelereqz, _Leclercqz, for technical teehnieal assistanee. assistance.