Polarization in the cosmic microwave background

Cosmic microwave background (CMB) studies underpin our understanding of the universe and its history. Until recently, we have relied principally on CMB temperature observations to build our standard cosmological model, but today the field forges ahead into its next frontier - CMB polarization anistropy. Polarization measurements will furnish fresh and independent information on the primordial density perturbations and cosmological parameters, and they offer the exciting potential to detect primordial gravity waves, constrain dark energy and measure the neutrino mass scale. I review the science and long-term goals of CMB polarization measurements and discuss current results and future observational projects. A vigorous program of ground-based, suborbital and space-based (e.g., WMAP and Planck [2008]) experiments is guiding us towards a future space mission dedicated to high precision polarization measurements.