  Deep images taken with the Wide Field Channel of the Advanced Camera for
Surveys on board the Hubble Space Telescope provide the basis for study the
resolved stellar population of the M81 companion dwarf irregular galaxy
Holmberg IX. Based on color-magnitude diagrams the stellar population toward
Holmberg IX contains numerous stars with ages of <~200 Myr as well as older red
giant stars. By charting the spatial distribution of the red giant stars and
considering their inferred metallicities, we concluded that most of these older
stars are associated with M81 or its tidal debris. At least 20% of the stellar
mass in Holmberg IX was produced in the last ~200 Myr, giving it the youngest
stellar populations if any nearby galaxy. The location of Holmberg IX, its high
gas content, and its youthful stellar population suggests that it is a tidal
dwarf galaxy, perhaps formed during the last close passage of M82 around M81.
