Recent experimental and theoretical results on the optical nonlinearity associated with molecular reorientation in dye-doped liquids and liquid crystals are reviewed. The variation of intermolecular forces experienced by a photo-excited dye molecule and the resulting change of molecular-statistical parameters such as rotational friction and orientational energy appear to be at the core of this effect. Photoinduced intramolecular charge transfer could be the cause of such variation. The angular momentum conservation is also discussed.