  Relativistic jets are collimated outflows with speeds close to light speed,
which are associated with gamma-ray bursts (GRBs), active galactic nuclei
(AGNs), and so on. This article mainly overviews recent developments of
polarimetric studies of GRBs and their afterglows in the gamma-ray and optical
wavebands as well as the first detections of their radio polarization.
Polarimetric observations and theoretical modelings can address the emission
mechanism, magnetic field structure, and energetics of GRB jets and related
collisionless plasma physics. Some of the discussed key physics are common with
AGN jets. Furthermore, we mention that polarimetry of AGN jets and
protoplanetary disks may be a novel approach to search for ultra-light axion
dark matter.
