M. Rahimi, C. L. Reichardt

Since the first detection by the DASI experiment in 2002, measurements of the polarization of the cosmic microwave background (CMB) have grown into an important role in testing our understanding of conditions in the early universe and cosmology. The field has seen rapid experimental progress, driven in large part by the desire to make increasingly precise measurements of CMB polarization. Precise measurements of the CMB polarization anisotropies contain as much information as the CMB temperature anisotropy, and promise to unlock new tests of physics and the standard cosmological model. In this chapter, we first discuss how polarization is produced through Thomson scattering, after which types of polarization patterns are connected to the cosmological sources. Finally, we briefly discuss the experimental hardware that enables these measurements.