Premenstrual symptoms occur in approximately 75% to 85% of women. PMS that causes significant disruption of dailylife occurs in approximately 5% to 10% of women. PMDD, rig-orously diagnosed as outlined in the DSM-IV, affects only 3% to 5% of women. PMS and PMDD can begin with menar-che, but can also present later in life, even in a woman’s 40s, though this is often a reflection of the hesitancy of women to seek medical help for their symptoms. The expression or symptom dominance of these disorders dif-fers depending on ethnicity and culture. There is some evidence that the incidence of PMDD varies across cul-tures as well, with high rates in Mediterranean cultures and the Middle East and low rates in Asia. Twin studies also demonstrate concordance, implying a genetic contri-bution to the development of these disorders.