a1 Geneva University, Geneva Observatory, CH-1290 Versoix, Switzerland email: firstname.lastname@example.org
a2 Osservatorio Astronomico di Trieste Via G. B. Tiepolo 11, I - 34131 Trieste, Italia email: Cristina.Chiappini@obs.unige.ch
a3 Keele University, Keele Staffordshire ST5 5BG, United Kingdom email: email@example.com
a4 Joint Institute for Nuclear Astrophysics University of Notre Dame, Notre Dame, IN 46556, United States
We review general characteristics of massive stars, present the main observable constraints that stellar models should reproduce. We discuss the impact of massive star nucleosynthesis on the early phases of the chemical evolution of the Milky Way (MW). We show that rotating models can account for the important primary nitrogen production needed at low metallicity. Interestingly such rotating models can also better account for other features as the variation with the metallicity of the C/O ratio. Damped Lyman Alpha (DLA) systems present similar characteristics as the halo of the MW for what concern the N/O and C/O ratios. Although in DLAs, the star formation history might be quite different from that of the halo, in these systems also, rotating stars (both massive and intermediate) probably play an important role for explaining these features. The production of primary nitrogen is accompanied by an overproduction of other elements as 13C, 22Ne and s-process elements. We show also how the observed variation with the metallicity of the number ratio of type Ibc to type II supernovae may be a consequence of the metallicity dependence of the line-driven stellar winds.