Rotation of the CMB polarisation by foreground lensing

The weak lensing corrections to the CMB polarization and anisotropies are the most relevant non-linearities in the CMB power spectra. After a brief introduction I shall concentrate on the effect of rotation and show that the rotation of polarisation is a true physical effect which has to be taken into account at second order in perturbation theory. I shall clarify some inconsistencies on previous treatments of this rotation in the literature.