Seizures do not occur randomly. They tend to cluster in the majority of men and women with epilepsy. Seizure clusters, in turn, often show a periodicity. When the periodicity of seizure exacerbation aligns itself with that of the menstrual cycle, it is designated as catamenial epilepsy. The neuroactive properties of reproductive steroids and the cyclic variation in their serum concentrations are important pathophysiologic factors. Recent investigations have demonstrated and confirmed the existence of at least three patterns of catamenial seizure exacerbation: perimenstrual and periovulatory in ovulatory cycles and entire luteal phase in anovulatory cycles. A rational mathematical basis for the categorization of seizure exacerbation as catamenial epilepsy has been developed. It identifies approximately one third of women as having catamenial epilepsy. If seizures show hormonal sensitivity in their occurrence, they may also respond to hormonal treatment. Successful open label trials using cyclic natural progesterone supplement, depomedroxyprogesterone and gonadotropin-releasing hormone analogues in women and using testosterone with or without aromatase inhibitor in men have been reported. Prospective, randomized, placebo-controlled, double-blind investigations are warranted and under way.