The location of theoretical stellar models in the upper part of the HR Diagram (HRD) depends on a variety of poorly understood physical proc esses which may occur during the evolution of massive stars. Among the most important we find: mass loss, convective overshoot and opacity enhancement for temperatures around 106 °K. The effects of these phenomena have been recently investigated by several authors (Bertelli et al. 1983 and references therein), but, due to the large uncertainties still present in the theoretical formulation of these processes, only parametrized formulae are avaliable for model computations. Since the theoretical dis tribution of massive stars in the HRD turns out to be fairly sensitive to these parameters, the evolutionary scenario for massive stars is still far from being satisfactorily settled. On the other hand, due to mass loss, nuclear processed material is expected to be exposed at the surface of massive stars and then, the comparison between theoretical predictions and observations of the surface chemical composition of these objects can help in understanding their evolution and to set more stringent limits to the mentioned parameters.