The main instabilities induced by rotation in stellar interiors are described .
We derive from first principles the general equation describing the transport of the angular momentum .
The case of the transport of the chemical species is also discussed .
As long as the mass loss rates are not too important , meridional currents , by advecting angular momentum from the inner regions to the outer layers , accelerate the stellar surface during the Main Sequence phase .
A 9 M _ { \odot } stellar model at solar metallicity with an equatorial velocity at the beginning of the core H–burning phase equal to 340 km s ^ { -1 } reaches the break–up limit during the MS phase .
The model with an initial velocity of 290 km s ^ { -1 } approaches this limit without reaching it .
The models with 290 km s ^ { -1 } and 340 km s ^ { -1 } predict enhancements of the N/C ratio at the end of the MS phase equal to 2.8 and 3.2 times the initial value respectively .