Although amenorrhea may result from a number of different conditions, a systematic evaluation including a detailed history, physical examination, and laboratory assessment of selected serum hormone levels can usually identify the underlying cause. Primary amenorrhea, which by definition is failure to reach menarche, is often the result of chromosomal irregularities leading to primary ovarian insufficiency (e.g., Turner syndrome) or anatomic abnormalities (e.g., Müllerian agenesis). Secondary amenorrhea is defined as the cessation of regular menses for three months or the cessation of irregular menses for six months. Most cases of secondary amenorrhea can be attributed to polycystic ovary syndrome, hypothalamic amenorrhea, hyperprolactinemia, or primary ovarian insufficiency. Pregnancy should be excluded in all cases. Initial workup of primary and secondary amenorrhea includes a pregnancy test and serum levels of luteinizing hormone, follicle-stimulating hormone, prolactin, and thyroid-stimulating hormone. Patients with primary ovarian insufficiency can maintain unpredictable ovarian function and should not be presumed infertile. Patients with hypothalamic amenorrhea should be evaluated for eating disorders and are at risk for decreased bone density. Patients with polycystic ovary syndrome are at risk for glucose intolerance, dyslipidemia, and other aspects of metabolic syndrome. Patients with Turner syndrome (or variant) should be treated by a physician familiar with the appropriate screening and treatment measures. Treatment goals for patients with amenorrhea may vary considerably, and depend on the patient and the specific diagnosis.