Young stellar objects (YSOs) are reviewed with an emphasis on their global evolutionary properties. The main classification schemes of YSOs based on the observed continuum spectral energy distributions and the recent suggestion of using a bolometric temperature to characterize the evolutionary status of all YSOs will be first discussed. The impact of protostar theory on pre-main-sequence evolution will be then addressed. The revised evolution differs in significant aspects from the traditional one, in particular for stars more massive than the sun. A new H-R diagram is presented in which the observed distribution of young stars of low and intermediate mass is naturally explained. The major sources of uncertainties for the location of evolutionary tracks, and hence for mass and age estimates of YSOs, are also discussed together with the effects of mass accretion and the presence of companions.