  HI observations in the 21cm line have been a principal tool for investigating
the large-scale structure of the Milky Way. This review considers what was
learned in the first decade after the discovery of the 21cm line, and how that
knowledge has been expanded and refined in subsequent years. Topics include
spiral structure, the Galactic nucleus, the thickness of the HI layer, and
affairs in the outskirts of the Galaxy. New advances in instrumentation and
computing, and a broad attack on problems using information from all
wavelengths, are likely to keep HI studies of the Milky Way interesting for
years to come.
