A brief review of the methods to evaluate the strength of magnetic fields in blazars on some examples using observational data is presented.

Abstract. Recent research has made clear the exceptional importance of spectropolarimetry in the study of physical conditions in atmospheres and circumstellar envelopes of different kinds of stars, in galactic nuclei and to obtain knowledge about interstellar matter. The goal of this brief review is to illustrate the field with several examples and to present new revelations made using ground-based spectropolarimetry with large telescopes. I conclude with a short description of a newly proposed spectropolarimeter for the WSO/UV mission.