Despite its name, the accessory optic system (AOS) is a primary visual system. Not only do retinal ganglion cell axons constitute a major input to AOS nuclei, but the AOS contributes fundamentally to retinal image stabilization, which is important for the proper functioning of other visual systems. Although the AOS has been recognized anatomically for well over 100 years and is present in all vertebrate classes (Mai 1978), the first physiological studies were not reported until 1959 (Marg et al 1959). Until quite recently, progress in understanding AOS physiology was limited. Indeed, in the most recent review of the physiol ogy of the AOS, Marg (1973) expressed concern as to whether the AOS even had a normal function. In the intervening years substantial progress has been made in revealing both the anatomy and physiology of the AOS, and these advances are the focus of this review. After a brief survey of the gross anatomical features of the AOS, the following topics are treated: retinal ganglion cell morphology and distribution, efferents and nonvisual afferents of the AOS, visual response properties of AOS neurons and their interpretation, behavioral consequences of AOS lesions, and relations and comparisons be tween the AOS and an allied pretectal nucleus.