  In the past few years, astronomers have uncovered several very
low-temperature, metal-poor stars with halo or thick disk kinematics and
peculiar spectral and photometric properties, so-called ultracool subdwarfs.
These include the first examples of L subdwarfs - metal-poor analogs of the L
dwarf spectral class - and slightly metal-deficient T dwarfs. Ultracool
subdwarfs provide useful empirical tests of low temperature atmosphere and
evolutionary models, and are probes of the halo mass function down to and below
the (metal-dependent) hydrogen burning limit. Here I summarize the optical and
near-infrared spectroscopic properties of these objects, review recent research
results, and point out scientific issues of interest in this developing
subject.
