In many vertebrates, in particular man and primates, an important part, maybe most, of the information coming from the environment is collected and processed by vision. Then, these animals have developed both sophisticated visual systems and effective Eye Movement, or OculoMotor, Systems (OMS). Both in higher and lower animals, the main role of eye movement (EM) is to serve vision and ensure optimal conditions for it. Then EMs may be different in different animals, depending on the particular features and needs of the animal's visual system.