The authors conducted a critical review of the optokinetic nystagmus (OKN) literature to consider OKN and in particular, vertical OKN. The review provides an overview of OKN to aid understanding of the eye movements. It provides a review of the literature relating to neurological control of POKN showing the complexity of this and inter relations between cortical and sub cortical pathways. It considers horizontal OKN, vertical OKN and the effects of target change in OKN gain. There is strong evidence supporting symmetry of horizontal OKN in normal adults. There is little agreement on vertical OPKN asymmetry. Thus the authors suggest that the finding of asymmetrical vertical OKN should prompt investigation of potential neurological pathology.