  We review advances in low temperature detector (LTD) arrays for Cosmic
Microwave Background (CMB) polarization experiments, with a particular emphasis
on imaging arrays. We briefly motivate the science case, which has spurred a
large number of independent experimental efforts. We describe the challenges
associated with CMB polarization measurements and how these challenges impact
LTD design. Key aspects of an ideal CMB polarization imaging array are
developed and compared to the current state-of-the-art. These aspects include
dual-polarization-sensitivity, background-limited detection over a 10:1
bandwidth ratio, and frequency independent angular responses. Although existing
technology lacks all of this capability, today's CMB imaging arrays achieve
many of these ideals and are highly advanced superconducting integrated
circuits. Deployed arrays map the sky with pixels that contain elements for
beam formation, polarization diplexing, passband definition in multiple
frequency channels, and bolometric sensing. Several detector architectures are
presented. We comment on the implementation of both transition-edge-sensor
bolometers and microwave kinetic inductance detectors for CMB applications.
Lastly, we discuss fabrication capability in the context of next-generation
instruments that call for $\sim 10^6$ sensors.
