3D-chiral materials can exhibit optical activity, which manifests itself as circular birefringence, causing polarization rotation for linearly polarized waves, and circular dichroism, causing different attenuation of left- and right-handed circularly polarized waves. The former can be exploited to realize polarization rotators, while the latter can be used to realize circular polarizers. Optical activity is weak in natural chiral materials, but it can be enhanced by orders of magnitude in an artificial chiral materials, i.e., chiral metamaterials. Just like the perceived sense of twist of a helix is the same for opposite directions of observation, optical activity is the same for opposite directions of wave propagation.