Headache is a common chief complaint in acute settings. The diagnosis of acute headache can be challenging and should proceed in an orderly fashion. An important first step is to distinguish primary from secondary headaches. The approach is to seek “red flags” that suggest the possibility of secondary headache. If one of these features is identified, the physician must conduct the workup indicated by the red flag and, thereby, diagnose any secondary headache disorder that is present. In the absence of secondary headache, the clinician proceeds to diagnosing a primary headache disorder. In this clinical summary, we follow this approach to discuss the differential diagnosis of acute headaches.