Ultracool Subdwarfs: Subsolar Metallicity Objects Down to Substellar Masses

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.