In the present review we summarize direct and indirect evidence that the
massive close binary frequency is very large. We then discuss the binary
evolutionary processes and we present a general massive close binary
evolutionary scheme. Finally, we highlight the importance of massive close
binaries for population number synthesis and the chemical evolution of
galaxies.Comment: 21 pages, 6 figures, to appear in "Massive Stars: Formation,
Evolution, Internal Structure and Environnement", eds. M. Heydari-Malayeri
and J.-P. Zah
