Abstract Headache is among the most common of human maladies. So much so that it is generally (and often incorrectly) assumed to be understood, especially by doctors. The classification of headache, with formal definitions of different diagnostic entities, by the International Headache Society into (1) primary—occurring in the absence of external causes, and (2) secondary—some of which may have sinister cause, has greatly simplified the description, understanding, and management of this often challenging symptom. It also allows those headaches with serious or life-threatening consequences to be distinguished from other forms. This chapter examines the various types of headache, from primary headaches and migraine, and tension-type to secondary headache and other forms.