In this talk I will present the capabilities of multi-wavelength and high-energy polarimetry towards the understanding of AGN physics through the practical example of the recent results obtained during a campaign on blazar BL Lacertae (BL Lac) by IXPE. This campaign was complemented with multi-spectral-range observations with large millimeter, radio and optical polarimeters, resulting in the detection of the most polarized blazar ever observed. More interestingly, the combination of radio, optical and X-ray polarimetry has allowed us to identify, for the first time, the dominant process responsible for the X-ray emission of a blazar, ruling out hadronic processes as the main drivers of X-ray radiation from BL Lac. These results open the way to future multi-wavelength polarimetric studies with upcoming instruments for high-energy polarimetry such as NASA’s COSI satellite, and have strong synergies with VHE gamma-ray instruments, that we are using to extend this analysis to the gamma-ray regime. This synergy is particularly relevant at the moment thanks to the construction of the Cherenkov Telescope Array Observatory (CTAO), that will be the best ground-based gamma-ray observatory for the next 3 decades.