Two basic biological premises determine the success of replacement of degenerated photoreceptors by a technical implant. First, the neuronal network in the residual retina of patients selected for implantation must still be capable of processing technically generated signals. Secondly, the implant itself must be biocompatible with tissue, i.e. it may not itself induce further degeneration. Our studies in animal models with advanced retinal degeneration and with donor retinas of retinitis pigmentosa patients have shown that even after complete destruction of the photoreceptors and long periods of blindness, the inner retina in the macular area remains for the most part histologically intact, and that all neurons are demonstrably still present and capable of successfully transmitting and processing signals. Biocompatibility of subretinal implants was studied in pigs. After 14 months of implantation, histological examination of tissue covering the implant showed that the inner retina was completely intact. There were no signs of histopathologic changes.