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.

Preface.- 1. History of HVC Research ¿ an overview; Bart P. Wakker, Klaas S. de Boer, Hugo van Woerden.- 2. HVC/IVC maps and HVC distribution functions; Bart P. Wakker.- 3. Kinematics of high- and intermediate-velocity gas; Ulrich J. Schwarz, Klaas S. de Boer.- 4. Intermediate-velocity clouds; C. Elise Albert, Laura Danly.- 5. HVCS related to the Magellanic System; Mary E. Putman.- 6. High-velocity HI gas in External Galaxies; Tom Oosterloo.- 7. The large- and small-scale structure of HVCS; Ulrich J. Schwarz, Bart P. Wakker.- 8. Ionized gas associated with HVCS; Stephen L. Tufte.- 9. The coldest phase in halo HVCS: Dust and molecules; Philipp Richter, Klaas S. de Boer.- 10. Distances and Metallicities; Hugo van Woerden, Bart P. Wakker.- 11. The hot halo; Klaas S. de Boer.- 12. HVCS interacting with their environment; Christian Brüns, Ulrich Mebold.- 13. Warps, polar rings and High-velocity clouds; Linda S. Sparke.- 14. High-velocity clouds, the missing link? Leo Blitz, David N. Spergel, Peter J. Teuben, Dap Hartmann.- 15. Compact, isolated high-velocity clouds; W. Butler Burton, Robert Braun, Vincent de Heij.- 16. The origin of the high-velocity clouds; Joel N. Bregman.- 17. Unsolved mysteries of high-velocity clouds; Robert A. Benjamin.- Index