One of the major targets for next-generation cosmic microwave background (CMB) experiments is the precision mapping of CMB distortions due to the gravitational lensing effect of dark matter. Estimating this lensing is important for two reasons. First, lensing probes the nature of dark matter fluctuations in the sky. Second, lensing estimates can be used, in principle, to de-lense the observed CMB for reconstructing the primordial CMB fluctuations. Both of these problems are crucial for next-generation CMB experiments.