The recent launch of the Imaging X-ray Polarimetry Explorer, commonly known as IXPE, has opened a completely new observing window for active galactic nuclei (AGNs). By observing, for the first time ever, the 2-8 keV polarization from AGNs, it became possible to try to solve several key problems of X-ray generation in AGNs : where do X-rays come from in blazars ? How are they generated ? Is there really an X-ray corona above the accretion disks and where is it situated ? What is its true geometry, composition and temperature ? During this talk, I will first introduce the concept of polarimetry, give a detailed description of the IXPE mission and give a long, in-depth review of the discoveries achieved by X-ray polarimetry in the field of AGNs.