  Among all the astronomical sources investigated through the prism of
polarimetry, active galactic nuclei (AGN) have proven to be the richest in
terms of complex yet fundamental signatures that helped to understand their
true nature. Indeed, AGN exhibit a wide range of wavelength-dependent
polarimetric features that are intrinsically related to their multi-scale
emission mechanisms. Each waveband is characterized by a different set of
polarimetric signatures that can be related to various physical mechanisms
that, from the radio band to the soft-$\gamma$ rays, probe increasingly smaller
AGN regions. In fact, panchromatic polarization measurements are the key to
understand how and when AGN form, accrete, and impact the host galaxy they
reside in. In this chapter, I will first introduce AGN without focusing on a
particular observational technique. I will then review the discoveries and
constraints that spectro-, imaging and broadband polarimetry have achieved.
Finally, I will highlight the important questions that remain unanswered and
how they can be solved with future large millimeter and radio antennas, 30-m
class optical and infrared telescopes, and high energy satellites equipped with
state-of-the-art polarimeters.
