  We describe a self-consistent spectrum analysis technique employing non-LTE
line formation, which allows precise atmospheric parameters of massive stars to
be derived: 1sigma-uncertainties as low as ~1% in effective temperature and
~0.05-0.10 dex in surface gravity can be achieved. Special emphasis is given to
the minimisation of the main sources of systematic errors in the atmospheric
model computation, the observed spectra and the quantitative spectral analysis.
Examples of applications are discussed for OB-type stars near the main sequence
and their evolved progeny, the BA-type supergiants, covering masses of ~8 to 25
Msun and a range in effective temperature from ~8000 to 35000 K. Relaxing the
assumption of local thermodynamic equilibrium in stellar spectral synthesis has
been shown to be decisive for improving the accuracy of quantitative analyses.
Despite the present examples, which concentrate on hot, massive stars, the same
philosophy can be applied to line-formation calculations for all types of
stars, including cooler objects like the Sun, once the underlying stellar
atmospheric physics is reproduced consistently.
