  In this paper, we review the current theory of very low mass stars model
atmospheres including the coolest known M~dwarfs, M~subdwarfs, and brown
dwarfs, i.e. T$_{eff} \leq 5,000 $K and $-2.0 \le [M/H] \le +0.0$. We discuss
ongoing efforts to incorporate molecular and grain opacities in cool stellar
spectra, as well as the latest progress in deriving the effective temperature
scale of M~dwarfs. We also present the latest results of the models related to
the search for brown dwarfs.
