  The maturity of current detectors based on technologies that range from solid
state to gases renewed the interest for X-ray polarimetry, raising the
enthusiasm of a wide scientific community to improve the performance of
polarimeters as well as to produce more detailed theoretical predictions. We
will introduce the basic concepts about measuring the polarization of photons,
especially in the X-rays, and we will review the current state of the art of
polarimeters in a wide energy range from soft~to hard X-rays, from solar flares
to distant astrophysical sources. We will introduce relevant examples of
polarimeters developed from the recent past up to the panorama of upcoming
space missions to show how the recent development of the technology is allowing
reopening the observational window of X-ray polarimetry.
