Ovarian hyperstimulation syndrome (OHSS) is characterized by ovarian enlargement and a shift of fluid from the intravascular to the extravascular space. OHSS is classified into mild OHSS, moderate OHSS, and severe OHSS. Vascular endothelial growth factor is a powerful mediator of vessel permeability. Angiogenin may play a role in neovascularization leading to the development of OHSS. The different complications of OHSS are vascular complications, liver dysfunction, renal complications, respiratory complications, and gastrointestinal complications. Current management of OHSS relies on the prediction and active prevention. Any patient undergoing ovarian stimulation is at risk of OHSS but it appears to be more frequent in younger women (aged less than 35 years) and women with polycystic ovarian syndrome (PCOS). This chapter discusses oestradiol monitoring of ovulation, ultrasonographic monitoring of OHSS, and prevention and treatment of OHS that includes basic treatment and surgical treatment.