On the occasion of the retirement of Ulrich Schwarz, a symposium was held in Groningen in May of 1996, celebrating his contributions to the study of the int- stellar medium, including his work on the high-velocity clouds. The coming together of many specialists in the latter ?eld prompted the idea of compiling a book c- taining their contributions, and summarizing the status of our understanding of the high-velocity cloud phenomenon. This seemed especially worthwhile at the time, since many exciting developments were taking place. After the discovery of some H i clouds with high velocities, about 40 years ago, the subject had been dominated by 21-cm observations of H i emission. Starting in the mid-1980s much progress was being made because of the availability of new instruments, such as large ground-based optical telescopes and UV observatories in space. The connections between the work on high-velocity clouds and other studies of the properties of the (hot) interstellar medium also became clearer.

"This book contains 17 chapters reviewing our knowledge of the high-velocity clouds (HVCs) as of 2004, bringing this together in one place for the first time. Each of the many different aspects of HVC research is addressed by one of the experts in that subfield. These include a historical overview of HVC research and analyses of the structure and kinematics of HVCs.

History of HVC research - an Overview.- HVC/IVC Maps and HVC Distribution Functions.- Kinematics of HV and IV Gas.- Interemdiate-velocity Clouds.- HVCS Related to the Magellanic System.- High-velocity Hi Gas in External Galaxies.- The Large- and Small-scale Structure of HVCs.- Ionized Gas Associated with HVCs.- The Coldest Phase in Halo High-velocity Gas: Dust and Molecules.- Distances and Metallicities of HVCS.- The Hot Halo.- HVCS Interacting With Their Environment.- Warps, Polar Rings and High-velocity Clouds.- High-Velocity Clouds: The Missing Link?.- Compact, Isolated High-velocity Clouds.- The Origin of the High-velocity Clouds.- Unsolved Mysteries of High-velocity Clouds.