Polycystic ovary syndrome (PCOS) is a highly prevalent endocrine disorder affecting 1 out of 8 women of reproductive age worldwide. Diagnostic features of PCOS include a combination of ovulatory dysfunction, hyperandrogenism and/or polycystic ovarian morphology (PCOM), with the most severe phenotype of PCOS involving a combination of all three cardinal features. As a broad-spectrum endocrine disorder, PCOS is associated with a host of metabolic complications that are integral to the pathophysiology of anovulation in PCOS. Mechanisms of disordered folliculogenesis in PCOS are largely unexplored however, and the degree to which PCOM captures the severity of anovulation is unclear. Overweight and obesity are highly prevalent in women with PCOS and have been shown to confer a detrimental impact on antral follicle development, ovulation and PCOS severity. First-line therapy for those with PCOS and excess weight is lifestyle intervention aimed at mild to moderate weight loss in the range of 5 – 10 % owing to the known benefits for metabolic health. The effectiveness of lifestyle intervention on ovulatory dysfunction and other diagnostic criteria for PCOS remains unclear. In Chapter 1, the effectiveness of weight loss on antral follicle development and ovulation in women with PCOS was evaluated using a short-term hypocaloric dietary intervention. We found that clinically meaningful weight loss did not consistently improve ovulation frequency. However, improvements in earlier stages of antral follicle development were observed suggesting that a longer intervention or greater weight loss could improve reproductive outcomes in the longer term. In Chapter 2, we evaluated the impact of a short-term hypocaloric dietary intervention on the severity of PCOS presentation. We showed that weight loss induced by a short-term hypocaloric dietary intervention resulted in changes in menstrual cycle length and antral follicle populations that were not sufficient to drive phenotypic changes in the majority of women with PCOS. Of those who experienced phenotypic changes in clinical presentation of PCOS, shifts to a more favorable PCOS phenotype were temporary and largely resolved 6 months later. In Chapter 3, we established the diagnostic accuracy of polycystic ovarian morphology across the menstrual cycle. We found that markers of follicle excess and ovarian enlargement on ultrasonography have high predictive power for PCOS across the early follicular phase, late follicular phase, early luteal phase, and late luteal phase. Diagnostic accuracy for PCOS was enabled by the persistent nature of polycystic ovarian morphology across the menstrual cycle. Together, this research will inform best practices for the diagnostic evaluation and nutritional management of PCOS