While optical activity is normally observed for transmitted light, polarization rotation and different attenuation of left-handed and right-handed circularly polarized waves can also occur for light reflected by chiral substances. These phenomena of specular circular birefringence and specular circular dichroism are jointly known as specular optical activity. Specular optical activity is weak in natural materials. Extrinsic 3d chirality associated with oblique illumination of metasurfaces lacking two-fold rotational symmetry leads to large specular optical activity.