Ovarian hyperstimulation syndrome (OHSS) is an iatrogenic complication of ovulation induction and ovarian stimulation for assisted reproductive technology (ART) and is characterized by cystic enlargement of the ovaries and rapid fluid shifts from the intravascular compartment to the third space. Six major severity based classifications of OHSS which have developed over a period of time are tabulated in this chapter. The exact etiology for the pathogenesis of OHSS is still unknown, but the syndrome is known to be dependent on human chorionic gonadotropin (hCG). The ovaries which are primed with gonadotropins and subsequently exposed to hCG lead to pathophysiology of increased vascular permeability. A detailed discussion on factors implicated in the past in pathogenesis of OHSS has been presented in this chapter. Identifying at-risk patients is critical in prevention of OHSS as it assists the clinician in making changes in ovarian stimulation protocols or taking other preventive measures. Risk factors can be primary or secondary. The clinical manifestations of OHSS are a cascade of pathophysiologic events resulting from increased vascular permeability. The management depends on the severity of OHSS and guides the clinician whether it should be done on an outpatient basis or admission is required.