The retinoid cycle (also referred to as the visual cycle) is the process by which the visual chromophore 11-cis-retinal (11cRAL) is released from light-activated opsins in the form all-trans-retinal and isomerized back to its 11-cis isomer ready for another photoisomerization reaction. This process involves oxidation, reduction and isomerization reactions and take place in the retinal pigment epithelium (RPE) and photoreceptor segments of the eye (von Lintig 2012, Blomhoff & Blomhoff 2006, von Lintig et al. 2010, D'Ambrosio et al. 2011). This section describes the retinoid cycle in rods during dark/twilight conditions.