Abstract: As they near the end of their lives, low-to-intermediate mass stars undergo copious mass-loss, leading to the formation of extensive circumstellar envelopes (CSEs) up to a parsec or more across. These envelopes are a primary source of dust and heavy element enrichment in the Galaxy, affect the structure of the interstellar medium on small scales, and are precursors to the formation of planetary nebulae. I will present results from a program aimed at improving our understanding of the evolutionary histories of mass-losing giants through a previously little-used probe of circumstellar material: the HI 21-cm line of neutral hydrogen. Because HI is not destroyed by the interstellar radiation field, it can trace CSEs to significant distances from the star, thereby probing mass-loss over very extended timescales (>100,000 years). HI also supplies unique kinematic information on the interaction between CSEs and their environments. Our results to date include the discovery of extended tails, bow shocks, and other structures associated with well-known evolved stars as well as new constraints on mass-loss timescales. We have recently expanded our HI studies to Cepheid variables, and I will also report on implications for mass-loss on the Cepheid instability strip as a means of resolving the long-standing "Cepheid mass discrepancy".