  Ultra-cool dwarfs have been defined as dwarfs with spectral types of M7 or
later (Kirkpatrick, Henry, & Irwin 1997) and thus include the new L and T
spectral classes. The previous IAU report on ultra-cool dwarfs in 1999
described the optical and IR spectral features that characterize these spectral
types. In this report, I review the latest progress on the spectral
classification of ultra-cool dwarfs, which are divided into two categories: old
dwarfs in the field (>1 Gyr) and their young progenitors in the nearest
star-forming regions and open clusters (1-100 Myr). Because the former are
nearby and the latter are young, these two groups are relatively bright and
lend themselves to discovery and detailed study. Technically, the luminosity
class of young ultra-cool objects is closer to subgiant than dwarf;
nevertheless, I include them in this report on ultra-cool dwarfs.
