Headache is the fourth most common reason for emergency department encounters, accounting for 3% of all visits in the United States. Though troublesome, 90% are relatively benign primary headaches --migraine, tension, and cluster headaches. The other 10% are secondary headaches, caused by separate underlying processes, with vascular, infectious, or traumatic etiologies, and they are potentially life-threatening. This issue details the important pathophysiologic features of the most common types of life-threatening headaches, the key historical and physical examination information emergency clinicians must obtain, the red flags that cannot be missed, and the current evidence for best-practice testing, imaging, treatment, and disposition.