Resolved Stellar Populations as Tracers of Outskirts
  Galaxy haloes contain fundamental clues about the galaxy formation and
evolution process: hierarchical cosmological models predict haloes to be
ubiquitous, and to be (at least in part) the product of past merger and/or
accretion events. The advent of wide-field surveys in the last two decades has
revolutionized our view of our own Galaxy and its closest "sister", Andromeda,
revealing copious tidal streams from past and ongoing accretion episodes, as
well as doubling the number of their known faint satellites. The focus shall
now be shifted to galaxy haloes beyond the Local Group: resolving individual
stars over significant areas of galaxy haloes will enable estimates of their
ages, metallicities and gradients. The valuable information collected for
galaxies with a range of masses, morphologies and within diverse environments
will ultimately test and quantitatively inform theoretical models of galaxy
formation, and shed light onto the many challenges faced by simulations on
galactic scales.
