The visual cycle (or phototransduction) is a fundamental bio-chemical process in the retina: it allows photoreceptors to convert light into electrical signals and subsequently to return to the dark state. George Wald obtained the Nobel Prize in 1967 for his pioneering studies on this process and it has been an active field of research in Ophthalmology ever since. I will discuss the key aspects of the visual cycle in photoreceptor cells and present a new mathematical model for the visual cycle in rod cells. The model consists of a system of coupled ODEs and PDEs for the concentrations of relevant molecules and proteins in rod outer segments. The goal is to give a quantitative description of the kinetics of the main photo-sensitive molecules after exposure to light. I will explain how the model can be extended in order to account for the accumulation of toxic byproducts in the eye in connection to degenerative retinal diseases.