We discuss the impact of the inclusion of both mass loss and rotation in massive star models at different metallicities. Rotation much improves the agreement between theory and observation. In particular rotational mixing allows to produce variations of the surface abundances already during the Main-Sequence phase. The changes of the surface abundances are more important when, for a given initial velocity, the initial mass increases, and/or the initial metallicity decreases. Also the observed high number of red supergiants at low metallicity can be accounted for by rotating stellar models, as well as the observed variation of the number ratio of Wolf-Rayet to O-type stars as a function of the metallicity and the variation with the metallicity of the number ratio of type Ibc to type II supernovae. In all these questions non-rotating models give unsatisfactory fits. Rotating models can also give interesting insights on questions as the origin of the Be stars, the mechanisms responsible for the huge mass loss rates undergone by the Luminous Blue Variables, the sources of primary nitrogen at low metallicity, the rotation rate of pulsars and the progenitors of the collapsars.