Objective: To develop and translate rigorous, comprehensive evidence-based guidelines for diagnosis, assessment, and treatment, to improve the lives of those with polycystic ovary syndrome (PCOS) worldwide. Here we will focus on diagnosis. Participants: Extensive health professional and consumer or patient engagement informed the guideline priority areas. International society-nominated panels included consumers, and experts in paediatrics, endocrinology, gynaecology, primary care, reproductive endocrinology, psychology, dietetics, exercise physiology, sleep, bariatric/metabolic surgery, public health, other co-opted experts, project management, evidence synthesis and translation. Evidence: Best practice, evidence-based guideline development involved extensive evidence synthesis and the Grading of Recommendations, Assessment, Development and Evaluation (GRADE) framework covered evidence quality, feasibility, acceptability, cost, implementation and ultimately recommendation strength. Process: Governance included an international advisory board, project board and five guideline development groups with 52 members, consumer, and translation committees. The Centre for Research Excellence in Women’s Health in Reproductive Life, funded by the Australian National Health and Medical Research Council (NHMRC), and led by Monash University, partnered with the American Society for Reproductive Medicine, the Endocrine Society, the European Society of Endocrinology and the European Society of Human Reproduction and Embryology. Thirty-six organisations collaborated with international meetings over nine months. Fifty-five prioritised clinical questions involved 52 systematic and three narrative reviews, generating evidence-based and consensus recommendations with accompanying practice points. Committee members nominated by collaborating organisations provided international peer review, and evidence expert-reviewed methods and outputs were submitted to NHMRC for independent review. Recommendations: PCOS should be diagnosed using the revised consensus Rotterdam criteria, which are now updated to evidence-based criteria in the International Guideline. In adults this requires the presence of two of i) clinical/biochemical hyperandrogenism ii) ovulatory dysfunction and iii) polycystic ovaries on ultrasound or elevated anti-mullerian hormone (AMH) levels, after other causes of these features are excluded. Where irregular menstrual cycles and hyperandrogenism are present, ultrasound or AMH are not required for diagnosis. In adolescents, both hyperandrogenism and ovulatory dysfunction are required, with ultrasound and AMH not recommended, due to poor specificity. Conclusion: Overall, evidence in PCOS is low to moderate quality. Based on high prevalence and significant health impact, greater priority, funding, and research is recommended. Guideline translation will be extensive including multilingual education outputs and evidence-based resources for consumers (the ASKPCOS app), healthcare professionals and policy makers.