Cloudy with a Chance of Iron: The Transition
Between Spectral Types L and T

Adam J. Burgasser
U.C.L.A.

The search for brown dwarfs has unveiled two new
spectral classes, L and T dwarfs, the first
additions to the main sequence in over 65 years.
Substellar objects evolve through both spectral
types as they cool, eventually becoming too faint to
detect with current technology.  We are now
beginning to see that the transition between these
two classes is complex, influenced by the rainoout
of heavy metals (Fe, V, Ti) and dust grains, and
modulated by cloud structure that may be
heterogeneous and can vary over time.  I present new
results for late L dwarfs and early T dwarfs that
highlight this transition, and discuss how the
observations may be explained (or not!) in light of
recent theoretical work.  I also describe some of
the implications of this transition on
classification schemes and the local substellar mass
function