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.

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.