The visual system has a pivotal role in most human activities. Many health disorders and psychomotor deficits involve the visual system at some level. From this some therapists infer a causal relation and apply a variety of visual therapies and trainings to cure a variety of deficits. The most prominent example is dyslexia, a disorder not caused by a defect in the primary visual system. Worldwide various non-evidence based therapies are used to treat dyslexia. The great number of exercises and visual trainings is contrasted by scientific evidence, which shows that exercises are useful in only few and selected disorders. The human visual system seems to be optimized such that no improvement can be achieved with training.