Polycystic ovary syndrome (PCOS) is a disorder whose definition and diagnosis have been evolving over the past 85 years since Stein and Leventhal made their original description of the triad of amenorrhea, polycystic ovaries, and hyperandrogenism in 1935. Although much progress in characterizing the disorder was accomplished from the late 1950s through the late 1980s, much confusion remained in the field, as there were no agreed-upon diagnostic criteria. This deficiency began to be addressed with a conference held at the National Institutes of Health (NIH) in April 1990, which, through a survey of meeting participants, defined what is today known as “classic” PCOS. Following the NIH 1990 conference, many of our European colleagues, who relied more exclusively on the ultrasound detection of polycystic ovaries for the diagnosis of PCOS, organized a second expert conference in Rotterdam in May 2003, where the diagnostic criteria were expanded. Finally, the Androgen Excess Society (now the Androgen Excess and PCOS Society) published its recommendations for diagnosis in November 2006, principally rooted in associating the criteria for PCOS with the risks of metabolic and other long-term morbidities. A breakthrough in our understanding of PCOS came when we realized that PCOS could be classified into 4 phenotypes (A–D) based on 3 features (clinical and/or biochemical hyperandrogenism, oligoanovulation, and polycystic ovaries) and that the Rotterdam 2003 and the Androgen Excess Society 2006 criteria were simply expansions of the NIH 1990 criteria. Having a clearer definition of PCOS has allowed us to establish PCOS as a globally prevalent disorder, affecting between 1 in 10 and 1 in 7 women, the most common endocrine disorder of women, and the most common cause of infertility, among other findings. However, it is critical that we understand that much of the work on the criteria and definition of PCOS has arisen through the process of scientific dialog, debate, and “consensus,” although many would argue that “consensus science” is not science at all. Despite the significant progress made over the past 50 years, much remains to be studied and advocated for when it comes to the care of women with PCOS.