  Arguments for including in the VO large grids of synthetic spectra fully
covering the HR diagram are presented. One obvious need is that of population
synthesis at all redshifts. Theoretical spectra have also the power to predict
peculiar behaviors, that could remain unnoticed in observed spectra, or lead to
erroneous conclusions. One interesting example is given of the Ca II H & K
lines in extremely metal-poor stars. In carbon-rich atmospheres, the H & K
lines become much weaker relative to the continuum, which will lead to an
underestimate of the metallicity. What actually happens is a displacement of
the continuum, obvious in absolute-flux synthetic spectra, but not visible in
observed continuum-normalized spectra. Libraries of synthetic spectra are
described, as well as the codes used to compute them. A few remarks are made on
issues with making these libraries available through the VO, as well as on the
necessary input data, i.e. line lists and model atmospheres.
