  The weak lensing effect on the cosmic microwave background (CMB) induces
distortions in spatial pattern of CMB anisotropies, and statistical properties
of CMB anisotropies become a weakly non-Gaussian field. We first summarize the
weak lensing effect on the CMB (CMB lensing) in the presence of scalar, vector
and tensor perturbations. Then we focus on the lensing effect on CMB statistics
and methods to estimate deflection angles and their power spectrum. We end by
summarizing recent observational progress and future prospect.
