HI and Galactic Structure

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.