Observations of the cosmic microwave background (CMB) are a powerful probe of the physics of the early universe and a robust discriminant of cosmological models.  Although what we observe on large scales are mainly the perturbations on the last-scattering surface, the CMB also contains imprints of the processes that occurred between the last-scattering surface and the present. Among such processes is weak gravitational lensing, which consists of the deflection of CMB photons by the mass of matter clustered along the line of sight. The gravitational weak lensing distorts the temperature-temperature (TT) correlation power spectrum by a few arcminutes but coherently over degree scales, which means that lensing becomes an important effect for $ellge 3000.$ Weak lensing also mixes and distorts the EE and BB polarisation power spectra. It also correlates with other processes which track the large scale structure. We discuss estimators of the weak lensing potential and report recent results on the extraction of the weak lensing from maps with excised pixels. We will also discuss the new challenges for precision cosmology that the new experiments present to us.