The prehospital patient with altered mental status (AMS) presents a challenge to EMS clinicians, who must rapidly and thoroughly assess and stabilize vital functions in an undifferentiated patient. Reversible causes must be evaluated and treated, and life-threatening conditions recognized. EMS clinicians are better able to effectively manage AMS patients when they understand the breadth of potentially concomitant causes of the condition. An organized approach to assessment and management of the AMS patient is important as a broad differential diagnosis is considered and an appropriate treatment pathway is pursued.