Ovarian hyperstimulation syndrome (OHSS) is an iatrogenic potentially life-threatening condition resulting from an excessive ovarian stimulation. The crucial event in the development of the OHSS is the administration of human chorionic gonadotropin (hCG). The early onset of OHSS typically presents in the luteal phase as a consequence of ovulatory hCG. The late onset of OHSS presents in the early gestation phase in which endogenous hCG is produced. There are different grades of OHSS. Many strategies have been proposed for OHSS prevention; however, they may reduce but not eliminate the risk. This chapter describes the OHSS pathophysiology, revises the diagnostic criteria for the diagnosis of OHSS, and presents the related preventative strategies available in the literature.