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.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.