This chapter deals with the classification, pathophysiology, prediction, prevention, and treatment of ovarian hyperstimulation syndrome (OHSS). OHSS is characterized by bilateral cystic ovarian enlargement and third-space fluid shift resulting in ascites and pleural effusion. It may be moderate or severe in severity, early or late in onset, spontaneous or iatrogenic in etiology. Prediction of OHSS is the cornerstone of prevention. It is based on identifying the characteristics of the patients who would be high responders as well as the use of ultrasonography and estradiol assessment. The pathophysiology of OHSS suggests the involvement of an inflammatory mechanism during the development of the fluid leakage associated with OHSS. Therefore, investigators hypothesized that glucocorticoids could possibly prevent OHSS in patients at high risk. Laparoscopic ovarian drilling has been used successfully for prevention of OHSS in patients with polycystic ovaries. The medical treatment of OHSS consists of correction of circulatory volume and electrolyte imbalance.