Retinitis Pigmentosa (RP) is the name given to a group of hereditary diseases of the retina of the eye. The retina lies at the back of the eye and it acts like the film in a camera, receiving and processing everything you see. The retina is a delicate layer of cells which picks up the pictures and transmits them to the brain. In humans there are two types of light sensitive cells in the retina: rod cells and cone cells.