This review addresses the methodologic issues relevant to the study of comorbidity and provides a broad overview of the medical and psychiatric conditions associated with migraine. Since the body of literature examining the comorbidity of migraine is enormous, the scope of this review will largely focus on community and case-control studies. Among the selected comorbid medical conditions, including disorders of the cardiovascular, respiratory, neurologic, gastrointestinal and immunologic systems, stroke, asthma, and allergies appear to be most strongly associated with migraine. Among the psychiatric illnesses, mood and anxiety disorders have been shown to be most strongly associated with migraine in the general population. Further research is indicated and necessary to elucidate the precise mechanism of migraine comorbidity.