  Brown dwarfs bridge the gap between the stellar and planetary mass regimes.
Evolving from conditions very similar to the lowest-mass stars, the atmospheres
of older brown dwarfs closely resemble those expected in close-in extrasolar
giant planets, and with cooler BDs still being discovered, more and more
approach the properties of gas giants at wider separation. Interpreting the
spectra of BDs is therefore a crucial step towards understanding and predicting
the spectral and thermal properties of EGPs.
  Essential properties of substellar atmospheres are massive molecular
line-blanketing and the condensation of species with decreasing Teff, changing
the chemical equilibrium composition and causing absorption from dust grains.
More complex details involve the distribution of dust clouds over the surface
giving rise to temporal variability, and possible deviations from chemical
equilibrium conditions. In the case of close-in EGPs and some BDs in binary
systems, the effect of irradiation from the primary significantly affects the
spectral properties and thermal evolution.
