  Recent discoveries from red optical proper motion and wide-field
near-infrared surveys have uncovered a new population of ultracool subdwarfs,
metal-poor stars and brown dwarfs extending into the late-type M, L and
possibly T spectral classes. These objects are among the first low-mass stars
and brown dwarfs formed in the Galaxy, and are valuable tracers of metallicity
effects in low-temperature atmospheres. Like solar metallicity late-type
dwarfs, ultracool subdwarfs emit the majority of their emergent flux at
infrared wavelengths. Here I discuss how Spitzer observations will contribute
to the study of these objects, enabling measurement of the temperature scale,
tests for dust formation and the identification of surface gravity and
metallicity diagnostics in the coldest brown dwarfs.
