The conventional wisdom about menstruation in adolescents, perpetuated in textbooks, requires updating. Recently published national surveys and reviews of large historical databases provide information about menstruation in adolescents, including the following: (1) Girls are experiencing earlier pubertal development than previously noted, suggesting that guidelines for the evaluation of potentially pathologic precocious puberty be reassessed. (2) There are racial differences in pubertal development, with African American girls experiencing earlier signs than Caucasian girls, and Mexican American girls intermediate in pace. (3) The absence of pubertal development by age 14 is associated with a high probability of conditions with impaired reproductive potential. (4) Absence of menarche by age 15 is statistically uncommon and should be evaluated. (5) Parameters for normal menstrual cyclicity indicate that most menstrual cycles for adolescents are between approximately 20 and 45 days. (6) Because menstrual cycles outside of this range are statistically uncommon, consideration should be give to evaluating adolescents with bleeding that is either too frequent or too infrequent. (7) A number of conditions with the potential for significant sequelae in adulthood can present as abnormal menses in adolescence, and thus merit early diagnosis and management.