longer excitable and proper transduction would not be possible. The normal transepithelial transport of ions would be too slow to compensate quickly enough for these changes, there are many underlying mechanisms based on the activity of voltage-dependent ion channels add to the basic transepithelial transport of ions. Visual cycle: The visual cycle fulfills an essential task of maintaining visual function and needs therefore to be adapted to different visual needs such as vision in darkness or lightness. For this, functional aspects come into play: the storage of retinal and the adaption of the reaction speed. Basically vision at low light intensities requires a lower turn-over rate of the visual cycle whereas during light the turn-over rate is much higher. In the transition from darkness to light suddenly, large amount of 11-cis retinal is required. This comes not directly from the visual cycle but from several retinal pools of retinal binding proteins which are connected to each other by the transportation and reaction steps of the visual cycle. Phagocytosis of photoreceptor outer segment (POS) membranes: POS are exposed to constant photo-oxidative stress, and they go through constant destruction by it. They are constantly renewed by shedding their end, which RPE then phagocytose and digest. Secretion: The RPE is an epithelium which closely interacts with photoreceptors on one side but must also be able to interact with cells on the blood side of the epithelium, such as endothelial cells or cells of the immune system. In order to communicate with the neighboring tissues the RPE is able to secrete a large variety of factors and signaling molecules. It secretes ATP, fas-ligand (fas-L), fibroblast growth factors (FGF-1, FGF-2, and FGF-5), transforming growth factor-β (TGF-β), insulin-like growth factor-1 (IGF-1), ciliary neurotrophic factor (CNTF), platelet-derived growth factor (PDGF), vascular endothelial growth factor (VEGF), lens epithelium-derived growth factor (LEDGF), members of the interleukin family, tissue inhibitor of matrix metalloproteinase (TIMP) and pigment epithelium-derived factor (PEDF). Many of these signaling molecules have important physiopathologic roles. Immune privilege of the eye: The inner eye represents an immune privileged space which is disconnected from the immune system of the blood stream. The immune privilege is supported by the RPE in two ways. First, it represents a mechanical and tight barrier which separates the inner space of the eye from the blood stream. Second, the RPE is able to communicate with the immune system in order to silence immune reaction