  Polarization measurements of the gamma-ray component of transient sources are
of great scientific interest, they are however, also highly challenging. This
is due to the typical low signal to noise and the potential for significant
systematic errors. Both issues are made worse by the transient nature of the
events which prompt one to observe a large portion of the sky. The POLAR
instrument was designed as a dedicated transient gamma-ray polarimeter. It made
use of a large effective area and large field of view to maximize the signal to
noise as well as the number of observed transients. Additionally, it was
calibrated carefully on ground and in orbit to mitigate systematic errors. The
main scientific goal of POLAR was to measure the polarization of the prompt
emission of Gamma-Ray Bursts. During the 6 months operation in orbit POLAR
observed 55 Gamma-Ray Bursts of which 14 were bright enough to allow for
constraining polarization measurements. In this chapter we mainly discuss about
the POLAR instrument along with the calibration and analysis procedures. Two
analyses are described, the first is a straightforward method previously
implemented in polarization measurements, whilst the second was developed to
improve the sensitivity and to mitigate several of the issues with the former.
Both methods are described in detail along with information on how these can be
extended to perform time and energy resolved polarization measurements.
