The appearance of a patient with a dilated pupil in the ophthalmologist’s or neurologist’s office raises the possibility of an acute neurologic emergency (Fig. 1.1); however, this is very rarely the case. An isolated pupillary abnormality is more often than not a benign event, with much more ophthalmologic than neurologic significance. On the other hand, pupillary involvement accompanied by other neurologic findings (Figs 1.2 and 1.3), obvious or subtle, may portend a very serious neurologic problem. A practical, intelligent approach to evaluating pupillary abnormalities will save the patient unnecessary procedures and anxiety. Examination of the pupil also helps to differentiate the various causes of visual dysfunction. A relative afferent pupillary defect (RAPD) is the sine qua non of optic nerve dysfunction and its presence or absence helps differentiate visual loss due to optic neuropathies from maculopathies and occipital lobe pathology.