  We present preliminary non-grey model atmospheres and interiors for cool
brown dwarfs. The resulting synthetic spectra are compared to available
spectroscopic and photometric observations of the coolest brown dwarf yet
discovered, Gl229B (Nakajima \etal, 1995). Despite recognized shortcomings of
current methane opacities, we find that the model spectra provide an excellent
fit to most of the spectral features of the brown dwarf. We confirm the
presence of methane absorption and the substellar nature of Gl229B. These
preliminary models set an upper limit for the effective temperature of 1000~K.
We also compute the evolution of brown dwarfs with solar composition and masses
from 0.02 to 0.065 $\msol$. While uncertainties in the age of the system yield
some undetermination for the mass of Gl229B, the most likely solution is
$m\approx 0.04-0.055 \msol$. In any case, we can set an upper limit $m=
0.065\msol$ for a very unlikely age $t=10$ Gyr.
