Neuro-opthalmology can appear an intimidating subject, which can put off many general practitioners, but with a basic knowledge of the simple pathways controlling pupil size, animals presenting with different sized pupils (anisocoria) can be approached with confidence. This lecture will cover the neurologic control of pupil size and how to determine which pupil is the abnormally-sized one, before summarising a logical approach to unilateral miosis and unilateral mydriasis. The focus will be on Horner syndrome and oculomotor nerve dysfunction, but less common causes for anisocoria will also be discussed.