  In the last two decades about a dozen methods were invented which derive,
from a series of composite spectra over the orbit, the spectra of individual
components in binary and multiple systems. Reconstructed spectra can then be
analyzed with the tools developed for single stars. Eventually this has created
the opportunity for chemical composition studies in previously inaccessible
components of binary stars, and to follow their chemical evolution, an
important aspect in understanding evolution of stellar systems. First, we
review new developments in techniques to separate and reconstruct individual
spectra, and thereafter concentrate on some applications. In particular, we
emphasize the elemental abundance studies for high-mass stars, and present our
recent results in probing theoretical evolution models which include effects of
rotationally induced mixing.
