Incomplete understanding of menstruation may place girls at risk for sexually transmitted diseases (STDs) and unintended pregnancy. Prior research suggests that European American and African American girls incompletely understand menstruation, yet little is known about menstrual knowledge in other ethnic groups. Using audiotaped focus group and individual interviews with 73 African American, Mexican American, Arab American, and European American girls, we assessed girls' menstrual understanding. Responses included reproduction, growing up, cleansing, messages about femininity, and not knowing. We found ethnic differences in the prominence of these themes. We learned that social and cultural factors play an important role in transmission of menstrual knowledge.