During the past 20 yearsin vitrofertilization (IVF) has become the treatment of choice for an increasing number of infertile couples. An understanding of the physiologic basis for this advanced procedure is critical to the appropriate selection of patients, choice of stimulation protocols, and use of adjuvant therapies. In addition, an understanding of the pathophysiology of the ovarian hyperstimulation syndrome may allow novel treatment of this potentially life-threatening iatrogenic complication.