Headache is a common reason for seeking medical attention. Most cases are benign primary headache disorders; however, there is significant overlap between symptoms of these disorders and secondary headaches. Differentiating these clinical scenarios requires a careful history with attention to red flag symptoms and a neurologic examination. These details can identify dangerous disorders: subarachnoid hemorrhage, reversible cerebral vasoconstriction syndrome, elevated intracranial pressure, hydrocephalus, cerebral venous sinus thrombosis, arterial dissection, central nervous system infection, and inflammatory vasculitis. Older, pregnant, or immunocompromised patients have a higher risk for secondary disorders; clinicians should have a different threshold to conduct evaluations in such patients.