  We analyse pre-Main Sequence evolutionary tracks for low mass stars with
masses $m \le 1.4 \msol$ based on the Baraffe et al. (1998) input physics. We
also extend the recent Chabrier et al. (2000) evolutionary models based on
dusty atmosphere to young brown dwarfs down to one mass of Jupiter. We analyse
current theoretical uncertainties due to molecular line lists, convection and
initial conditions. Simple tests on initial conditions show the high
uncertainties of models at ages $\simle$ 1 Myr. We find a significant
sensitivity of atmosphere profiles to the treatment of convection at low
gravity and $\te < 4000$ K, whereas it vanishes as gravity increases. This
effect adds another source of uncertainty on evolutionary tracks at very early
phases. We show that at low surface gravity ($\log g \simle 3.5$,) the common
picture of vertical Hayashi lines with constant $\te$ is oversimplified. The
effect of a variation of initial deuterium abundance is studied. We compare our
models with evolutionary tracks available in the literature and discuss the
main differences. We finally analyse to which extent current observations of
young systems provide a good test for pre-Main Sequence tracks.
