  The aim of this article is to demonstrate the useful role that can be played
by spectropolarimetric observations of young and evolved emission line stars
that analyse the linearly polarized component in their spectra. At the time of
writing, this demonstration has to be made on the basis of optical data since
there is no common-user infrared facility, in operation, that offers the
desired combination of spectral resolution and sensitivity. Here we focus on
what can be learned from linear spectropolarimetry alone at reasonably high
spectral resolution and at $10^3 < $S/N$ < 10^4$. And we remind that the near
infrared (1--2 micron) has the potential to out-perform the optical as a domain
to work in because of the greatly reduced interstellar obscuration at these
wavelengths. This point has been reached at a time when theory, exploiting
flexible Monte Carlo methods, is fast becoming a powerful tool. In short we
have the complex phenomena, and the rise of the modelling capability to match
-- good data are the missing link.
