This chapter reviews landmark endocrine trials that have fundamentally shaped the management of diabetes, osteoporosis, and hormone therapy in postmenopausal women. Foundational glycemic control studies such as UKPDS, ACCORD, ADVANCE, and VADT clarified the role and limitations of intensive glucose lowering on microvascular and macrovascular outcomes. Prevention-focused studies like the DPP demonstrated the superiority of structured lifestyle interventions over pharmacotherapy for delaying diabetes onset. Cardiovascular outcomes trials, including EMPA-REG OUTCOME, LEADER, and SUSTAIN-6, established SGLT2 inhibitors and GLP-1 receptor agonists as therapies that extend beyond glycemic control to reduce cardiovascular and renal events, reshaping treatment guidelines worldwide. Beyond diabetes, the WHI trial overturned decades of routine hormone replacement therapy by showing net harm in chronic disease prevention, while the FIT trial provided definitive evidence that bisphosphonates like alendronate reduce vertebral and hip fracture risk in postmenopausal women with osteoporosis. Together, these trials underscore the central principle of modern endocrinology: therapy must extend beyond biochemical targets to address patient-centered outcomes such as cardiovascular health, fracture prevention, and overall survival.