AbstractA knowledge of the molecular structure and the chemistry of photoreceptors is essential in order to understand how they function. Chloroplasts and the outer segments of the retinal rods and cones of the eye are photoreceptors; they are able to receive, convert and transfer light energy to chemical and to electrical energy in the processes of photosynthesis, vision and nervous excitation. Chemically, their major constitutents are pigments, lipids and proteins. Structurally, they are an ordered system of tightly‐packed plates, discs, or tubes. Their fine structure in electron micrographs appears as electron dense double layers (lamellae) with a total thickness of the order of 250 Ɨ; each lamella or membrane at the interface is of the order of 50Ɨ.To discover how the pigment molecules are associated with these lamellae in the photoreceptors, the geometry (that is, the length, diameter, and number of lamellae) was measured and the pigment concentration determined spectroscopically. These data were then used to calculate the area that each pigment molecule would occupy if spread as a monolayer on the lamellar surfaces. In addition, pigment‐lipid protein micelle structures have also been studied.On the basis of these physical‐chemical studies, molecular models for the photoreceptors have been proposed.