From an observational point of view, magnetic field leaves its imprint, via Zeeman effect, on spectral line splitting and the induced wavelength dependence of the polarization state of the observed photons. The characteristics of the most recent polarimetric analyzers are described, together with their limitations. Next, inversion techniques are presented as the most accurate diagnostic tools available at present to determine the properties of magnetic field, derived from its polarization signature. Finally, the importance of feeding inversion codes with the adequate physics are stressed.