  The Milky Way is acquiring gas from infalling high-velocity clouds. The
material enters a disk-halo interface that in many places is populated with HI
clouds that have been ejected from the disk through processes linked to star
formation. The Smith Cloud is an extraordinary example of a high-velocity cloud
that is bringing $>10^6$ M$_{\odot}$ of relatively low metallicity gas into the
Milky Way. It may be part of a larger stream, components of which are now
passing through the disk.
