This thesis is composed of four clinical trials which examine diagnostic, prognostic and management aspects of pleural malignancy and pleural infection, all contributing novel data to the existing body of knowledge in these disease areas. The studies presented are as follows: • The South West Area Mesothelioma and Pemetrexed (SWAMP) Trial - a prospective observational trial examining markers of prognosis and chemotherapy response in malignant pleural mesothelioma. • A study of the association between plasma and pleural fluid Vascular Endothelial Growth Factor (VEGF) and its soluble receptor (sVEGFR-1) and survival and pleurodesis outcomes in a consecutive series of patients with a malignant pleural effusion . • A prospective observational study of serum and pleural fluid mesothelin testing in the investigation of undiagnosed pleural effusions. • Pleural irrigation trial (PIT): A randomised controlled pilot study of pleural irrigation with normal saline versus standard care in patients with pleural infection.