The pupil-to-light reflex includes direct-to-light reflex and indirect-to-light reflex. The direct reflex is performed as follows: if the pupil is rapidly narrowed by using a light source close to the irradiated pupil, it is called the direct reflex; if the pupil on the opposite side is narrowed, it is called the indirect reflex. It is best to examine the pupil’s reflex from the distant side of the patient, so that the patient does not gaze at the near flashlight, which may cause the same pupil narrowing after the adjustment reflex. The pupil-to-light reflex has a very important clinical role to play in determining the pathology of a disease. If the pupillary reflex is not sensitive, there is a high degree of suspicion that an intracranial lesion has occurred, such as actinic nerve palsy, which may result in ptosis, dilated pupils, and loss of the light reflex. In patients with end-stage brain herniation the pupils are dilated and all light reflexes are also absent.