  We present evolutionary models for low mass stars and brown dwarfs ($m \le
1.2 \msol$) based on recent improvement of the theory: equation of state,
atmosphere models, ... We concentrate on early evolutionary phases from the
initial deuterium burning phase to the zero-age Main Sequence. Evolutionary
models for young brown dwarfs are also presented. We discuss the uncertainties
of the present models. We analyse the difficulties arising when comparing
models with observations for very young objects, in particular concerning the
problem of reddening.
