The document summarizes observations and modeling of HI in circumstellar envelopes around asymptotic giant branch (AGB) stars. It discusses using HI observations to study the stellar winds and surrounding material due to HI's high velocity resolution and abundance. As a case study, it examines observations of the AGB star Y CVn using CO lines, dust continuum, and HI to determine properties of its circumstellar shell like expansion velocity and distance from the star. Modeling is then used to fit the observational data and learn about the shell.