Premenstrual syndromes (PMS) are a group of menstrually related, chronic, cyclical disorders manifested by emotional and physical symptoms in the second half of the menstrual cycle. A diagnosis of PMS is based on the timing and symptom pattern observed in daily symptom reports maintained by the patient. The etiology is unknown but is believed to involve genetic and dynamically evolving vulnerability, reproductive hormones, and neurotransmitters, as well as other brain processes. Effective medications have been identified for acute hormonal and symptomatic treatment of the disorder, but the time course and frequency of relapse and recurrence remain poorly understood. Further studies of PMS in all areas--etiology, pathobiology, diagnosis, course of illness, and treatment efficacy--are needed to increase the body of information about the menstrual cycle's effects on women's health and mood disorders.