Headaches are considered the most common type of pain; more than 40 million Americans seek treatment each year. A clear understanding of the types and possible causes of headache pain is essential to adequately assess and manage the patient with headaches. Headaches can be categorized as either primary or secondary to an underlying and usually treatable cause. Most headaches are primary; this includes migraine and variants, and cluster and tension-type headches. Before assuming a primary diagnosis, however, the clinician must rule out headaches secondary to an underlying cause so that further investigation, treatment, or referral may be inititated.