  Using the Advanced Camera for Surveys on the Hubble Space Telescope, we have
obtained deep optical images reaching stars well below the oldest main sequence
turnoff in the spheroid, tidal stream, and outer disk of the Andromeda Galaxy.
We have reconstructed the star formation history in these fields by comparing
their color-magnitude diagrams to a grid of isochrones calibrated to Galactic
globular clusters observed in the same bands. Each field exhibits an extended
star formation history, with many stars younger than 10 Gyr but few younger
than 4 Gyr. Considered together, the star counts, kinematics, and population
characteristics of the spheroid argue against some explanations for its
intermediate-age, metal-rich population, such as a significant contribution
from stars residing in the disk or a chance intersection with the stream's
orbit. Instead, it is likely that this population is intrinsic to the inner
spheroid, whose highly-disturbed structure is clearly distinct from the
pressure-supported metal-poor halo that dominates farther from the galaxy's
center. The stream and spheroid populations are similar, but not identical,
with the stream's mean age being ~1 Gyr younger; this similarity suggests that
the inner spheroid is largely polluted by material stripped from either the
stream's progenitor or similar objects. The disk population is considerably
younger and more metal-rich than the stream and spheroid populations, but not
as young as the thin disk population of the solar neighborhood; instead, the
outer disk of Andromeda is dominated by stars of age 4 - 8 Gyr, resembling the
Milky Way's thick disk. The disk data are inconsistent with a population
dominated by ages older than 10 Gyr, and in fact do not require any stars older
than 10 Gyr.
