Distances and metallicities of high-velocity clouds

Distance and metallicity hold the key to our understanding of the physical properties and origins of HVCs. We review the problems and results of determinations of distance and metallicity based on the presence or absence, and strength, of appropriate absorption lines in the spectra of stars at different distances and of extragalactic background sources. Upper or lower distance limits are available for several HVCs, and indicate various locations with respect to the major structural components of the Galaxy. The only distance bracket available,