After reviewing the historic differentiation between migraine and tension-type headache, the authors note that the similarities between these two types of primary headaches outweigh the differences, and so hypothesize that these headaches share a common pathophysiology. The convergence hypothesis for primary headaches links the clinical features of an evolving headache to current pathophysiological models. The authors suggest that successive symptoms experienced clinically reflect an escalating pathophysiological process, beginning with the premonitory period and progressing into tension-type headache and, if uninterrupted, finally into migraine. The clinical manifestations of other headache types, such as so-called sinus headache or temporomandibular headache, may also be explained by this model. A convergence hypothesis for primary headaches has important implications for earlier recognition, diagnosis, and treatment.