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 (metaldependent) hydrogen burning limit. Here I summarize the optical and nearinfrared spectroscopic properties of these objects, review recent research results, and point out scientific issues of interest in this developing subject.