  This chapter reviews our current knowledge of metal-poor ultracool dwarfs
with spectral types later than M7. The current census of M, L, and T subdwarfs
is explored. The main colour trends of subdwarfs from the optical to the
mid-infrared are described and their spectral features presented, which led to
a preliminary and tentative spectral classification subject to important
changes in the future when more of these metal-poor objects are discovered.
Their multiplicity and the determination of their physical parameters
(effective temperature, gravity, metallicity, and mass) are discussed. Finally,
some suggestions and future guidelines are proposed to foster our knowledge on
the oldest and coolest members of our Galaxy.
