  I present the preliminary results of a program to measure the star formation
history in the halo of the Andromeda galaxy. Using the Advanced Camera for
Surveys (ACS) on the Hubble Space Telescope, we obtained the deepest optical
images of the sky to date, in a field on the southeast minor axis of Andromeda,
51 arcmin (11 kpc) from the nucleus. The resulting color-magnitude diagram
(CMD) contains approximately 300,000 stars and extends more than 1.5 mag below
the main sequence turnoff, with 50% completeness at V=30.7 mag. We interpret
this CMD using comparisons to ACS observations of five Galactic globular
clusters through the same filters, and through chi-squared fitting to a
finely-spaced grid of calibrated stellar population models. We find evidence
for a major (approximately 30%) intermediate-age (6-8 Gyr) metal-rich ([Fe/H] >
-0.5) population in the Andromeda halo, along with a significant old metal-poor
population akin to that in the Milky Way halo. The large spread in ages
suggests that the Andromeda halo formed as a result of a more violent merging
history than that in our own Milky Way.
