Invited review abstract

In this introductory talk, I will review the main properties of the early evolution of young objects, from brown dwarfs, to low-intermediate-high mass stars. Most of the observational constraints on the mass of these objects rely on the use of simplified models of constant mass evolution. However, gaining mass is a distrubing effect for all stars (metaphorically speaking), and real stars are no exception. Accretion, slow or fast, can modify the mass gained at birth during the PMS phase and affect the interpretation of the observations of some key diagnostics. A brief discussion on the derivation of the IMF in the Orion Nebula Cluster will close the presentation.