This chapter presents landmark clinical trials that have shaped the diagnosis, treatment, and prognosis of pulmonary diseases. The studies span asthma management strategies (BASALT trial), COPD pharmacotherapy and long-term outcomes (TORCH, UPLIFT, and the Lung Health Study), the role of CPAP in obstructive sleep apnea with cardiovascular comorbidity (SAVE trial), and diagnostic accuracy for pulmonary embolism (PIOPED study). Venous thromboembolism management is addressed through the EINSTEIN-DVT, EINSTEIN-PE, and EINSTEIN-Extension trials. Pulmonary arterial hypertension treatment and risk stratification are informed by the REVEAL registry and the AMBITION trial, while the ASCEND and CAPACITY trials provide pivotal evidence for antifibrotic therapy in idiopathic pulmonary fibrosis. Together, these trials have influenced modern pulmonary guidelines, refined risk assessment tools, expanded therapeutic options, and clarified when early intervention can alter both quality of life and survival for patients with respiratory disease.