Individually, both obesity and headache are conditions associated with a substantial personal and societal impact. Recent data support that obesity is comorbid with headache in general and migraine specifically, as well as with certain secondary headache conditions such as idiopathic intracranial hypertension. In the current manuscript, we first briefly review the epidemiology of obesity and common primary and secondary headache disorders individually. This is followed by a systematic review of the general population data evaluating the association between obesity and headache in general, and then obesity and migraine and tension-type headache disorders. Finally, we briefly discuss the data on the association between obesity and a common secondary headache disorder that is associated with obesity, idiopathic intracranial hypertension. Taken together, these data suggest that it is important for clinicians and patients to be aware of the headache/migraine-obesity association, given that it is potentially modifiable. Hypotheses for mechanisms of the obesity-migraine association and treatment considerations for overweight and obese headache sufferers are discussed in the companion manuscript, as part II of this topic.