  New surveys with the Spitzer space telescope identify distant star-forming
and active galaxies by their strong emission at far-infrared wavelengths, which
provides strong constraints on these galaxies' bolometric energy. Using early
results from Spitzer surveys at 24 micron, we argue that the faint sources
correspond to the existence of a population of infrared-luminous galaxies at z
> 1 that are not expected from predictions based on previous observations from
ISO and IRAS. Combining Spitzer images with deep ground-based optical and
Hubble Space Telescope imaging, we discuss the properties of galaxies selected
at 24 micron in the region of the Chandra Deep Field South, including redshift
and morphological distributions. Galaxies with z < 1 constitute roughly half of
the faint 24 micron sources. Infrared-luminous galaxies at these redshifts span
a wide variety of normal to strongly interacting/merging morphologies, which
suggests that a range of mechanisms produce infrared activity. Large-area,
joint surveys between Spitzer and HST are needed to understand the complex
relation between galaxy morphology, structure, environment and activity level,
and how this evolves with cosmic time. We briefly discuss strategies for
constructing surveys to maximize the legacy of these missions.
