The pupillary light reflex is an important component of the neurologic examination of the trauma victim. Although the normal reflex can be predictably altered by specific head injuries, a variety of other factors common to trauma patients such as alcohol, illicit drugs, narcotics, paralyzing agents, hypothermia, and orbital or ophthalmic injury can confound the evaluation of the pupillary light reflex. This report reviews the anatomy and neurophysiology of the pupillary light reflex and discusses the impact these confounding variables may have on this key component of the initial trauma evaluation.