Classically, the pupil light reflex pathway is considered to be a simple reflex arc consisting of the retinal ganglion cells, intercalated neurons in the midbrain, the oculomotor nerve, and short ciliary nerves. However, there are some specialties in the structure of the afferent pupillary pathway that should be taken into account when interpreting pupillary disorders and that can help in the topodiagnosis of the lesion. Moreover, studies in patients with lesions of the retrogeniculate pathway showed that the pupillary pathway is more complex than previously assumed and the retrogeniculate visual pathway and the visual cortex are also involved in the pupillary light reaction. Clear anatomic evidence is still lacking but pupillographic measurements in patients with various disorders of the visual pathway support the existence of two pupillomotor channels that drive the pupil light reaction – the subcortical (more primitive, luminance channel associated with the intrinsically photosensitive retinal ganglion cells) and the suprageniculate (responds to shifts in structured stimuli, is driven by the rods and cones, and receives input from the visual cortex and extrastriate areas). The chapter summarizes possible pupillary findings in patients with homonymous hemianopia.