Pre-Main Sequence models for low-mass stars and brown dwarfs

10 pages, Latex file, uses Cargesepasp.sty, to be published in ASP Conf. Series, \"From Darkness to Light\", eds. T. Montmerle and P. Andre, Cargese 2000International audienceWe 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