The C20 diterpenoid compound retinal is the chromophore of the visual pigments the rhodopsins, and the pigments present in Halobacterium halobium, namely, bacteriorhodopsin (proton pump), halorhodopsin (chloride pump), and the sensory rhodopsins (phototaxis receptor). In all cases, they are bound covalently to the receptor protein by a protonated Schiff base. However, in rhodopsins, the retinal is the 11-cis isomer, whereas in H. halobium pigments it is the all-trans isomer. Why did Nature choose retinal as the chromophore, and why 11-cis in some cases and all-trans in other cases? Also why is the chromophore a protonated Schiff base? These points are addressed after giving an outline of the current status of the various photoreceptor pigments