Above the convective core where mixing of chemicals is instantaneous and efficient, some layers can be affected by partial or total mixing during the main sequence phase of evolution. The extent of this "extra mixed zone" as well as the mixing efficiency are, however, difficult to assess. This additional mixing has very important consequences since it involves longer time scales for nuclear burning phases and may in particular affect the value of the stellar mass at the transition between those stars which end up their life as white dwarfs and those which face a final supernova explosion. The impact on the chemical evolution of the galaxy is obvious. Physical reasons for this extra-mixing are various, either a mixing induced by internal rotation or a mixing resulting from convective bubbles crossing the convective core boundary to enter the radiative zone where they finally lose their identity (overshooting), or even some other poorly known processes.