Ovarian hyperstimulation syndrome (OHSS) is a serious iatrogenic complication of ovarian stimulation using fertility medications. OHSS classification presents the different severity categories and grades of OHSS and optimizes management schemes and prognosis. An initial classification system, which grouped OHSS into mild, moderate and severe categories, was based on patients' symptomatology plus manual estimation of ovarian enlargement and urinary excretion of sex steroids. A revised classification system then also incorporated the use of transvaginal sonography for both estimation of ovarian enlargement and detection of even small amounts of ascitic fluid. The detection of ascites establishes the diagnosis of moderate OHSS which may deteriorate to a severe form and is therefore of major importance. Subsequent modifications defined a group of critical or complicated OHSS in which grave complications have already developed. A consensus on the classification of OHSS should be reached by professional societies. As the revised classification system is very valid and widely used, it should form the basis for a modern classification, with the addition of critical or complicated OHSS to the severe category of the syndrome.