Summary1. The relatively high refractive index of rod and cone outer segments causes them to show wave‐guide properties.2. Rod outer segments have a positive intrinsic birefringence and a negative form birefringence. The former results from oriented lipid molecules, the latter from the disk structure of the outer segment.3. Electron microscopy of outer segments shows that the disks are formed from infoldings of the surface membrane, each disk consisting of two folds adhering together on their outer surfaces. Studies of other cell membranes have suggested that their lipid molecules form a continuous or discontinuous bimolecular leaflet which is sandwiched between protein layers, and this structure probably applies to each of the disk membranes.4. From the lipid content of rod outer segments a rough estimate of the thickness of a continuous lipid layer in each disk membrane can be made. This is only a little smaller than the requirements of a bimolecular leaflet.5. Rhodopsin is a lipoprotein whose lipid content forms a considerable fraction of the total outer‐segment lipid. This and other data suggest that rhodopsin may be a structural component of the disk membrane.6. The initial effect of light on rhodopsin is probably to isomerize II‐cisto all‐transretinene, and various thermal rearrangements follow which eventually cause the detachment of retinene from opsin. The absorption spectrum of rhodopsin is similarin situand in solution, but the absorption spectra of the photoproducts are somewhat different.7. Rhodopsin is strongly dichroic in the rod, indicating that the retinene molecule lies mainly in the plane of the disks.8. Rhabdomeres, the photoreceptor organelles of many higher invertebrates, are formed from close‐packed microvilli. They show a negative birefringence which probably results from the lipid molecules within the microvillar membrane.9. All well‐authenticated rhabdomere visual pigments have a retinene chromophore, and in many features the photochemical processes resemble those of vertebrate rhodopsin. The extraction processes for vertebrate and invertebrate rhodopsins are also similar.10. The question of whether insect rhabdomeres show dichroism is still open, but there is good evidence for dichroism in some cephalopod rhabdomeres. This is probably the physiological basis for the polarized light discrimination shown byoctopus.11. Some possible mechanisms for the transmission of the light stimulus within photoreceptor organelles ar